LETTERS
ON
NATURAL MAGIC,
ADDRESSED TO
SIR WALTER SCOTT, BART.

LETTER III.

Subject of spectral illusions—Recent and interesting case of Mrs. A.—Her first illusion affecting the ear—Spectral apparition of her husband—Spectral apparition of a cat—Apparition of a near and living relation in grave-clothes, seen in a looking-glass—Other illusions, affecting the ear—Spectre of a deceased friend sitting in an easy-chair—Spectre of a coach-and-four filled with skeletons—Accuracy and value of the preceding cases—State of health under which they arose—Spectral apparitions are pictures on the retina—The ideas of memory and imagination are also pictures on the retina—General views of the subject—Approximate explanation of spectral apparitions.

The preceding account of the different sources of illusion to which the eye is subject is not only useful as indicating the probable cause of any individual deception, but it has a special importance in preparing the mind for understanding those more vivid and permanent spectral illusions to which some individuals have been either occasionally or habitually subject.

In these lesser phenomena, we find the retina so powerfully influenced by external impressions, as to retain the view of visible objects long after they are withdrawn: we observe it to be so excited by local pressures of which we sometimes know neither the nature nor the origin, as to see in total darkness moving and shapeless masses of coloured light; and we find, as in the case of Sir38 Isaac Newton, and others, that the imagination has the power of reviving the impressions of highly luminous objects, months and even years after they were first made. From such phenomena, the mind feels it to be no violent transition to pass to those spectral illusions which, in particular states of health, have haunted the most intelligent individuals, not only in the broad light of day, but in the very heart of the social circle.

This curious subject has been so ably and fully treated in your Letters on Demonology, that it would be presumptuous in me to resume any part of it on which you have even touched; but as it forms a necessary branch of a Treatise on Natural Magic, and as one of the most remarkable cases on record has come within my own knowledge, I shall make no apology for giving a full account of the different spectral appearances which it embraces, and of adding the results of a series of observations and experiments on which I have been long occupied, with the view of throwing some light on this remarkable class of phenomena.

A few years ago, I had occasion to spend some days under the same roof with the lady to whose case I have above referred. At that time she had seen no spectral illusions, and was acquainted with the subject only from the interesting volume of Dr. Hibbert. In conversing with her about the cause of these apparitions, I mentioned, that if she should ever see such a thing, she might distinguish a genuine ghost, existing externally, and seen as an external object, from one created by the mind, by merely pressing one eye or straining them both, so as to see objects double;39 for in this case the external object or supposed apparition would invariably be doubled, while the impression on the retina created by the mind would remain single. This observation recurred to her mind when she unfortunately became subject to the same illusions; but she was too well acquainted with their nature to require any such evidence of their mental origin; and the state of agitation which generally accompanies them seems to have prevented her from making the experiment as a matter of curiosity.

1. The first illusion to which Mrs. A. was subject was one which affected only the ear. On the 26th of December, 1830, about half-past four in the afternoon, she was standing near the fire in the hall, and on the point of going up stairs to dress, when she heard, as she supposed, her husband’s voice calling her by name, “—— Come here! come to me!” She imagined that he was calling at the door to have it opened, but upon going there and opening the door she was surprised to find no person there. Upon returning to the fire, she again heard the same voice calling out very distinctly and loudly, “—— Come, come here!” She then opened two doors of the same room, and upon seeing no person she returned to the fire-place. After a few moments she heard the same voice still calling, “—— ---- Come to me, come! come away!” in a loud, plaintive, and somewhat impatient tone. She answered as loudly, “Where are you? I don’t know where you are;” still imagining that he was somewhere in search of her: but receiving no answer, she shortly went up stairs. On Mr. A.’s return to the house, about half an hour40 afterwards, she inquired why he called to her so often, and where he was; and she was, of course, greatly surprised to learn that he had not been near the house at the time. A similar illusion, which excited no particular notice at the time, occurred to Mrs. A. when residing at Florence about ten years before, and when she was in perfect health. When she was undressing after a ball, she heard a voice call her repeatedly by name, and she was at that time unable to account for it.

2. The next illusion which occurred to Mrs. A. was of a more alarming character. On the 30th of December, about four o’clock in the afternoon, Mrs. A. came down stairs into the drawing-room, which she had quitted only a few minutes before, and on entering the room she saw her husband, as she supposed, standing with his back to the fire. As he had gone out to take a walk about half an hour before, she was surprised to see him there, and asked him why he had returned so soon. The figure looked fixedly at her with a serious and thoughtful expression of countenance, but did not speak. Supposing that his mind was absorbed in thought, she sat down in an arm-chair near the fire, and within two feet at most of the figure, which she still saw standing before her. As its eyes, however, still continued to be fixed upon her, she said, after the lapse of a few minutes, “Why don’t you speak,——?” The figure immediately moved off towards the window at the further end of the room, with its eyes still gazing on her, and it passed so very close to her in doing so, that she was struck by the circumstance of hearing no41 step nor sound, nor feeling her clothes brushed against, nor even any agitation in the air. Although she was now convinced that the figure was not her husband, yet she never for a moment supposed that it was anything supernatural, and was soon convinced that it was a spectral illusion. As soon as this conviction had established itself in her mind, she recollected the experiment which I had suggested, of trying to double the object: but before she was able distinctly to do this, the figure had retreated to the window, where it disappeared. Mrs. A. immediately followed it, shook the curtains and examined the window, the impression having been so distinct and forcible that she was unwilling to believe that it was not a reality. Finding, however, that the figure had no natural means of escape, she was convinced that she had seen a spectral apparition like those recorded in Dr. Hibbert’s work, and she consequently felt no alarm or agitation. The appearance was seen in bright daylight, and lasted four or five minutes. When the figure stood close to her it concealed the real objects behind it, and the apparition was fully as vivid as the reality.

3. On these two occasions Mrs. A. was alone, but when the next phantasm appeared her husband was present. This took place on the 4th of January, 1830. About ten o’clock at night, when Mr. and Mrs. A. were sitting in the drawing-room, Mr. A. took up the poker to stir the fire, and when he was in the act of doing this, Mrs. A. exclaimed, “Why there’s the cat in the room!” “Where?” asked Mr. A. “There, close to you,” she replied. “Where?” he repeated. “Why on the rug, to be sure, between42 yourself and the coal-scuttle.” Mr. A., who had still the poker in his hand, pushed it in the direction mentioned: “Take care,” cried Mrs. A., “take care, you are hitting her with the poker.” Mr. A. again asked her to point out exactly where she saw the cat. She replied, ”Why sitting up there close to your feet on the rug. She is looking at me. It is Kitty—come here, Kitty!”—There were two cats in the house, one of which went by this name, and they were rarely if ever in the drawing-room. At this time Mrs. A. had no idea that the sight of the cat was an illusion. When she was asked to touch it, she got up for the purpose, and seemed as if she were pursuing something which moved away. She followed a few steps, and then said, “It has gone under the chair.” Mr. A. assured her it was an illusion, but she would not believe it. He then lifted up the chair, and Mrs. A. saw nothing more of it. The room was then searched all over, and nothing found in it. There was a dog lying on the hearth, who would have betrayed great uneasiness if a cat had been in the room, but he lay perfectly quiet. In order to be quite certain, Mr. A. rang the bell, and sent for the two cats, both of which were found in the housekeeper’s room.

4. About a month after this occurrence, Mrs. A., who had taken a somewhat fatiguing drive during the day, was preparing to go to bed about eleven o’clock at night, and, sitting before the dressing-glass, was occupied in arranging her hair. She was in a listless and drowsy state of mind, but fully awake. When her fingers were in active motion among the papillotes, she was suddenly startled by seeing in the mirror the43 figure of a near relation, who was then in Scotland and in perfect health. The apparition appeared over her left shoulder, and its eyes met hers in the glass. It was enveloped in grave-clothes, closely pinned, as is usual with corpses, round the head, and under the chin, and though the eyes were open, the features were solemn and rigid. The dress was evidently a shroud, as Mrs. A. remarked even the punctured pattern usually worked in a peculiar manner round the edges of that garment. Mrs. A. described herself as at the time sensible of a feeling like what we conceive of fascination, compelling her for a time to gaze on this melancholy apparition, which was as distinct and vivid as any reflected reality could be, the light of the candles upon the dressing-table appearing to shine fully upon its face. After a few minutes, she turned round to look for the reality of the form over her shoulder; but it was not visible, and it had also disappeared from the glass when she looked again in that direction.

5. In the beginning of March, when Mr. A. had been about a fortnight from home, Mrs. A. frequently heard him moving near her. Nearly every night, as she lay awake, she distinctly heard sounds like his breathing hard on the pillow by her side, and other sounds such as he might make while turning in bed.

6. On another occasion, during Mr. A.’s absence, while riding with a neighbour, Mr.——, she heard his voice frequently as if he were riding by his side. She heard also the tramp of his horse’s feet, and was almost puzzled by hearing him address her at the same time with the person really in company. His voice made remarks on44 the scenery, improvements, &c., such as he probably should have done had he been present. On this occasion, however, there was no visible apparition.

7. On the 17th March, Mrs. A. was preparing for bed. She had dismissed her maid, and was sitting with her feet in hot water. Having an excellent memory, she had been thinking upon and repeating to herself a striking passage in the Edinburgh Review, when on raising her eyes, she saw seated in a large easy-chair before her the figure of a deceased friend, the sister of Mr. A. The figure was dressed as had been usual with her, with great neatness, but in a gown of a peculiar kind, such as Mrs. A. had never seen her wear, but exactly such as had been described to her by a common friend as having been worn by Mr. A.’s sister during her last visit to England. Mrs. A. paid particular attention to the dress, air, and appearance of the figure, which sat in an easy attitude in the chair, holding a handkerchief in one hand. Mrs. A. tried to speak to it, but experienced a difficulty in doing so; and in about three minutes the figure disappeared. About a minute afterwards, Mr. A. came into the room, and found Mrs. A. slightly nervous, but fully aware of the delusive nature of the apparition. She described it as having all the vivid colouring and apparent reality of life; and for some hours preceding this and other visions, she experienced a peculiar sensation in her eyes, which seemed to be relieved when the vision had ceased.

8. On the 5th October, between one and two o’clock in the morning, Mr. A. was awoke by Mrs. A., who told him that she had just seen the figure of his deceased mother draw aside the bed45curtains and appear between them. The dress and the look of the apparition were precisely those in which Mr. A.’s mother had been last seen by Mrs. A. at Paris, in 1824.

9. On the 11th October, when sitting in the drawing-room, on one side of the fire-place, she saw the figure of another deceased friend moving towards her from the window at the further end of the room. It approached the fire-place, and sat down in the chair opposite. As there were several persons in the room at the time, she describes the idea uppermost in her mind to have been a fear lest they should be alarmed at her staring, in the way she was conscious of doing, at vacancy, and should fancy her intellect disordered. Under the influence of this fear, and recollecting a story of a similar effect in your work on Demonology, which she had lately read, she summoned up the requisite resolution to enable her to cross the space before the fire-place, and seat herself in the same chair with the figure. The apparition remained perfectly distinct till she sat down, as it were, in its lap, when it vanished.

10. On the 26th of the same month, about two P.M., Mrs. A. was sitting in a chair by the window in the same room with her husband. He heard her exclaim—”What have I seen?” And on looking at her, he observed a strange expression in her eyes and countenance. A carriage-and-four had appeared to her to be driving up the entrance-road to the house. As it approached, she felt inclined to go up stairs to prepare to receive company, but, as if spellbound, she was unable to move or speak. The carriage approached, and as it arrived within a46 few yards of the window, she saw the figures of the postilions and the persons inside take the ghastly appearance of skeletons and other hideous figures. The whole then vanished entirely, when she uttered the above-mentioned exclamation.

11. On the morning of the 30th October, when Mrs. A. was sitting in her own room with a favourite dog in her lap, she distinctly saw the same dog moving about the room during the space of about a minute or rather more.

12. On the 3rd December, about nine P.M., when Mr. and Mrs. A. were sitting near each other in the drawing-room occupied in reading, Mr. A. felt a pressure on his foot. On looking up he observed Mrs. A.’s eyes fixed with a strong and unnatural stare on a chair about nine or ten feet distant. Upon asking her what she saw, the expression of her countenance changed, and upon recovering herself, she told Mr. A. that she had seen his brother, who was alive and well at the moment in London, seated in the opposite chair, but dressed in grave-clothes, and with a ghastly countenance, as if scarcely alive.

Such is a brief account of the various spectral illusions observed by Mrs. A. In describing them I have used the very words employed by her husband in his communications to me on the subject;5 and the reader may be assured that the descriptions are neither heightened by fancy, nor amplified by invention. The high character and intelligence of the lady, and the station of her husband in society, and as a man of learning and science, would authenticate the most marvellous narrative, and satisfy the most scrupulous47 mind, that the case has been philosophically as well as faithfully described. In narrating events which we regard as of a supernatural character, the mind has a strong tendency to give more prominence to what appears to itself the most wonderful; but from the very same cause, when we describe extraordinary and inexplicable phenomena which we believe to be the result of natural causes, the mind is prone to strip them of their most marvellous points, and bring them down to the level of ordinary events. From the very commencement of the spectral illusions seen by Mrs. A., both she and her husband were well aware of their nature and origin, and both of them paid the most minute attention to the circumstances which accompanied them, not only with the view of throwing light upon so curious a subject, but for the purpose of ascertaining their connection with the state of health under which they appeared.

As the spectres seen by Nicolai and others had their origin in bodily indisposition, it becomes interesting to learn the state of Mrs. A.’s health when she was under the influence of these illusions. During the six weeks within which the first three illusions took place, she had been considerably reduced and weakened by a troublesome cough, and the weakness which this occasioned was increased by her being prevented from taking a daily tonic. Her general health had not been strong, and long experience has put it beyond a doubt, that her indisposition arises from a disordered state of the digestive organs. Mrs. A. has naturally a morbidly sensitive imagination, which so painfully affects her corporeal impressions, that the account of any48 person having suffered severe pain by accident or otherwise, occasionally produces acute twinges of pain in the corresponding parts of her person. The account, for example, of the amputation of an arm will produce an instantaneous and severe sense of pain in her own arm. She is subject to talk in her sleep with great fluency, to repeat long passages of poetry, particularly when she is unwell, and even to cap verses for half an hour together, never failing to quote lines beginning with the final letter of the preceding one till her memory is exhausted.

Although it is not probable that we shall ever be able to understand the actual manner in which a person of sound mind beholds spectral apparitions in the broad light of day, yet we may arrive at such a degree of knowledge on the subject as to satisfy rational curiosity, and to strip the phenomena of every attribute of the marvellous. Even the vision of natural objects presents to us insurmountable difficulties, if we seek to understand the precise part which the mind performs in perceiving them; but the philosopher considers that he has given a satisfactory explanation of vision, when he demonstrates that distinct pictures of external objects are painted on the retina, and that this membrane communicates with the brain by means of nerves of the same substance as itself, and of which it is merely an expansion. Here we reach the gulf which human intelligence cannot pass; and if the presumptuous mind of man shall dare to extend its speculations farther, it will do it only to evince its incapacity and mortify its pride.

In his admirable work on this subject, Dr. Hibbert has shown that spectral apparitions are49 nothing more than ideas or the recollected images of the mind, which, in certain states of bodily indisposition, have been rendered more vivid than actual impression, or, to use other words, that the pictures in the “mind’s eye” are more vivid than the pictures in the body’s eye. This principle has been placed by Dr. Hibbert beyond the reach of doubt; but I propose to go much farther, and to show that the “mind’s eye” is actually the body’s eye, and that the retina is the common tablet on which both classes of impressions are painted, and by means of which they receive their visual existence according to the same optical laws. Nor is this true merely in the case of spectral illusions; it holds good of all ideas recalled by the memory or created by the imagination, and may be regarded as a fundamental law in the science of pneumatology.

It would be out of place in a work like this to adduce the experimental evidence on which it rests, or even to explain the manner in which the experiments themselves must be conducted: but I may state in general, that the spectres conjured up by the memory or the fancy have always a “local habitation,” and that they appear in front of the eye, and partake in its movements exactly like the impressions of luminous objects, after the objects themselves are withdrawn.

In the healthy state of the mind and body, the relative intensity of these two classes of impressions on the retina is nicely adjusted. The mental pictures are transient and comparatively feeble, and in ordinary temperaments are never capable of disturbing or effacing the direct images of visible objects. The affairs of life50 could not be carried on if the memory were to intrude bright representations of the past into the domestic scene, or scatter them over the external landscape. The two opposite impressions, indeed, could not co-exist: the same nervous fibre which is carrying from the brain to the retina the figures of memory, could not at the same instant be carrying back the impressions of external objects from the retina to the brain. The mind cannot perform two different functions at the same instant, and the direction of its attention to one of the two classes of impressions necessarily produces the extinction of the other: but so rapid is the exercise of mental power, that the alternate appearance and disappearance of the two contending impressions are no more recognized than the successive observations of external objects during the twinkling of the eyelids. If we look for example at the façade of St. Paul’s, and without changing our position call to mind the celebrated view of Mont Blanc from Lyons, the picture of the cathedral, though actually impressed upon the retina, is momentarily lost sight of by the mind, exactly like an object seen by indirect vision; and during the instant the recollected image of the mountain, towering over the subjacent range, is distinctly seen, but in a tone of subdued colouring and indistinct outline. When the purpose of its recall is answered, it quickly disappears, and the picture of the cathedral again resumes the ascendancy.

In darkness and solitude, when external objects no longer interfere with the pictures of the mind, they become more vivid and distinct; and51 in the state between waking and sleeping the intensity of the impressions approaches to that of visible objects. With persons of studious habits, who are much occupied with the operations of their own minds, the mental pictures are much more distinct than in ordinary persons; and in the midst of abstract thought, external objects even cease to make any impression on the retina. A philosopher absorbed in his contemplations experiences a temporary privation of the use of his senses. His children or his servants will enter the room directly before his eyes without being seen. They will speak to him without being heard; and they will even try to rouse him from his reverie without being felt; although his eyes, his ears, and his nerves actually receive the impressions of light, sound, and touch. In such cases, however, the philosopher is voluntarily pursuing a train of thought on which his mind is deeply interested; but even ordinary men, not much addicted to speculations of any kind, often perceive in their mind’s eye the pictures of deceased or absent friends, or even ludicrous creations of fancy, which have no connexion whatever with the train of their thoughts. Like spectral apparitions they are entirely involuntary, and though they may have sprung from a regular series of associations, yet it is frequently impossible to discover a single link in the chain.

If it be true, then, that the pictures of the mind and spectral illusions are equally impressions upon the retina, the latter will differ in no respect from the former, but in the degree of vividness with which they are seen; and those52 frightful apparitions become nothing more than our ordinary ideas, rendered more brilliant by some accidental and temporary derangement of the vital functions. Their very vividness, too, which is their only characteristic, is capable of explanation. I have already shown that the retina is rendered more sensible to light by voluntary local pressure, as well as by the involuntary pressure of the blood-vessels behind it; and if, by looking at the sun, we impress upon the retina a coloured image of that luminary, which is seen even when the eye is shut, we may by pressure alter the colour of that image, in consequence of having increased the sensibility of that part of the retina on which it is impressed. Hence we may readily understand how the vividness of the mental pictures must be increased by analogous causes.

In the case both of Nicolai and Mrs. A. the immediate cause of the spectres was a deranged action of the stomach. When such a derangement is induced by poison, or by substances which act as poisons, the retina is peculiarly affected, and the phenomena of vision are singularly changed. Dr. Patouillet has described the case of a family of nine persons who were all driven mad by eating the root of the hyoscyamus niger, or black henbane. One of them leapt into a pond, another exclaimed that his neighbour would lose a cow in a month, and a third vociferated that the crown piece of sixty pence would in a short time rise to five livres. On the following day they had all recovered their senses, but recollected nothing of what had happened. On the same day they all saw objects double, and, what is still more remarkable, on the third day53 every object appeared to them as red as scarlet. Now this red light was probably nothing more than the red phosphorescence produced by the pressure of the blood-vessels on the retina, and analogous to the masses of blue, green, yellow, and red light, which have been already mentioned as produced by a similar pressure in headaches, arising from a disordered state of the digestive organs.

Were we to analyse the various phenomena of spectral illusions, we should discover many circumstances favourable to these views. In those seen by Nicolai, the individual figures were always somewhat paler than natural objects. They sometimes grew more and more indistinct, and became perfectly white; and, to use his own words, “he could always distinguish with the greatest precision phantasms from phenomena.” Nicolai sometimes saw the spectres when his eyes were shut, and sometimes they were thus made to disappear,—effects perfectly identical with those which arise from the impressions of very luminous objects. Sometimes the figures vanished entirely, and at other times only pieces of them disappeared, exactly conformable to what takes place with objects seen by indirect vision, which most of those figures must necessarily have been.

Among the peculiarities of spectral illusions, there is one which merits particular attention, namely, that they seem to cover or conceal objects immediately beyond them. It is this circumstance more than any other which gives them the character of reality, and at first sight it seems difficult of explanation. The distinctness of any impression on the retina is entirely inde54pendent of the accommodation of the eye to the distinct vision of external objects. When the eye is at rest, and is not accommodated to objects at any particular distance, it is in a state for seeing distant objects most perfectly. When a distinct spectral impression, therefore, is before it, all other objects in its vicinity will be seen indistinctly, for while the eye is engrossed with the vision, it is not likely to accommodate itself to any other object in the same direction. It is quite common, too, for the eye to see only one of two objects actually presented to it. A sportsman who has been in the practice of shooting with both his eyes open, actually sees a double image of the muzzle of his fowling-piece, though it is only with one of these images that he covers his game, having no perception whatever of the other. But there is still another principle upon which only one of two objects may be seen at a time. If we look very steadily and continuously at a double pattern, such as those on a carpet composed of two single patterns of different colours, suppose red and yellow; and if we direct the mind particularly to the contemplation of the red one, the green pattern will sometimes vanish entirely, leaving the red alone visible; and by the same process the red one may be made to disappear. In this case, however, the two patterns, like the two images, may be seen together; but if the very same portion of the retina is excited by the direct rays of an external object, when it is excited by a mental impression, it can no more see them both at the same time, than a vibrating string can give out two different fundamental sounds. It is quite possible, however, that the brightest parts of a spectral figure may55 be distinctly seen along with the brightest parts of an object immediately behind it, but then the bright parts of each object will fall upon different parts of the retina.

These views are illustrated by a case mentioned by Dr. Abercrombie. A gentleman, who was a patient of his, of an irritable habit, and liable to a variety of uneasy sensations in his head, was sitting alone in his dining-room in the twilight, when the door of the room was a little open. He saw distinctly a female figure enter, wrapped in a mantle, with the face concealed by a large black bonnet. She seemed to advance a few steps towards him, and then stop. He had a full conviction that the figure was an illusion of vision, and he amused himself for some time by watching it; at the same time observing that he could see through the figure so as to perceive the lock of the door, and other objects behind it.6

If these views be correct, the phenomena of spectral apparitions are stripped of all their terror, whether we view them in their supernatural character, or as indications of bodily indisposition. Nicolai, even, in whose case they were accompanied with alarming symptoms, derived pleasure from the contemplation of them, and he not only recovered from the complaint in which they originated, but survived them for many years.—Mrs. A., too, who sees them only at distant intervals, and with whom they have but a fleeting existence, will, we trust, soon lose her exclusive privilege, when the slight indisposition which gives them birth has subsided.

56

LETTER IV.

Science used as an instrument of imposture—Deceptions with plane and concave mirrors practised by the ancients—The magician’s mirror—Effects of concave mirrors—Aërial images—Images on smoke—Combination of mirrors for producing pictures from living objects—The mysterious dagger—Ancient miracles with concave mirrors—Modern necromancy with them, as seen by Cellini—Description and effects of the magic lantern—Improvements upon it—Phantasmagoric exhibitions of Philipstall and others—Dr. Young’s arrangement of lenses, &c., for the Phantasmagoria—Improvements suggested—Catadioptrical phantasmagoria for producing the pictures from living objects—Method of cutting off parts of the figures—Kircher’s mysterious hand-writing on the wall—His hollow cylindrical mirror for aërial images—Cylindrical mirror for re-forming distorted pictures—Mirrors of variable curvature for producing caricatures.

In the preceding observations man appears as the victim of his own delusions—as the magician unable to exercise the spirits which he has himself called into being. We shall now see him the dupe of preconcerted imposture—the slave of his own ignorance—the prostrate vassal of power and superstition. I have already stated that the monarchs and priests of ancient times carried on a systematic plan of imposing upon their subjects—a mode of government which was in perfect accordance with their religious belief: but it will scarcely be believed that the same delusions were practised after the establishment of Christianity, and that even the Catholic sanctuary was often the seat of these unhallowed machinations.57 Nor was it merely the low and cunning priest who thus sought to extort money and respect from the most ignorant of his flock: bishops and pontiffs themselves wielded the magician’s wand over the diadem of kings and emperors, and, by the pretended exhibition of supernatural power, made the mightiest potentates of Europe tremble upon their thrones. It was the light of science alone which dispelled this moral and intellectual darkness, and it is entirely in consequence of its wide diffusion that we live in times when sovereigns seek to reign only through the affections of their people, and when the minister of religion asks no other reverence but that which is inspired by the sanctity of his office and the purity of his character.

It was fortunate for the human race that the scanty knowledge of former ages afforded so few elements of deception. What a tremendous engine would have been worked against our species by the varied and powerful machinery of modern science! Man would still have worn the shackles which it forged, and his noble spirit would still have groaned beneath its fatal pressure.

There can be little doubt that the most common, as well as the most successful, impositions of the ancients were of an optical nature, and were practised by means of plane and concave mirrors. It has been clearly shown by various writers that the ancients made use of mirrors of steel, silver, and a composition of copper and tin, like those now used for reflecting specula. It is also very probable, from a passage in Pliny, that glass mirrors were made at Sidon; but it is evident, that, unless the object presented to them was58 illuminated in a very high degree, the images which they formed must have been very faint and unsatisfactory. The silver mirrors, therefore, which were universally used, and which are superior to those made of any other metal, are likely to have been most generally employed by the ancient magicians. They were made to give multiplied and inverted images of objects, that is, they were plane, polygonal or many-sided, and concave. There is one property, however, mentioned by Aulus Gellius, which has given unnecessary perplexity to commentators. He states that there were specula, which, when put in a particular place, gave no images of objects, but when carried to another place, recovered their property of reflection.7 M. Salverte is of opinion that, in quoting Varro, Aulus Gellius was not sufficiently acquainted with the subject, and erred in supposing that the phenomenon depended on the place instead of the position of the mirror; but this criticism is obviously made with the view of supporting an opinion of his own—that the property in question may be analogous to the phenomenon of polarised light, which, at a certain angle, refuses to suffer reflexion from particular bodies. If this idea has any foundation, the mirror must have been of glass or some other body not metallic, or, to speak more correctly, there must have been two such mirrors, so nicely adjusted not only to one another, but to the light incident upon each, that the effect could not possibly be produced but by a philoso59pher thoroughly acquainted with the modern discovery of the polarisation of light by reflexion. Without seeking for so profound an explanation of the phenomenon, we may readily understand how a silver mirror may instantly lose its reflecting power in a damp atmosphere, in consequence of the precipitation of moisture upon its surface, and may immediately recover it when transported into drier air.

Fig. 3.

One of the simplest instruments of optical deception is the plane mirror, and when two are combined for this purpose it has been called the magician’s mirror. An observer in front of a plane mirror sees a distinct image of himself; but if two persons take up a mirror, and if the one person is as much to one side of a line perpendicular to the middle of it as the other is to the other side, they will see each other, but not themselves. If we now suppose MC, CD, NC, CD to be the partitions of two adjacent apartments let square openings be made in the partitions at A60 and B, above five feet above the floor, and let them be filled with plate glass, and surrounded with a picture frame, so as to have the appearance of two mirrors. Place two mirrors, E, F, one behind each opening at A and B, inclined 45° to the partition MN, and so large that a person looking into the plates of glass at A and B will not see their edges. When this is done it is obvious that a person looking into the mirror A will not see himself, but will see any person or figure placed at B. If he believes that he is looking into a common mirror at A, his astonishment will be great at seeing himself transformed into another person, or into any living animal that may be placed at B. The success of this deception would be greatly increased if a plane mirror, suspended by a pulley, could be brought immediately behind the plane glass at A, and drawn up from it at pleasure. The spectator at A, having previously seen himself in this moveable mirror, would be still more astonished when he afterwards perceived in the same place a face different from his own. By drawing the moveable mirror half up, the spectator at A might see half of his own face joined to half of the face placed at B; but in the present day the most ignorant persons are so familiar with the properties of a looking-glass, that it would be very difficult to employ this kind of deception with the same success which must have attended it in a more illiterate age. The optical reader will easily see that the mirror F and the apartment NCD are not absolutely necessary for carrying on this deception; for the very same effects will be produced if the person at B is stationed at G, and61 looks towards the mirror F in the direction GF. As the mirror F, however, must be placed as near to A as possible, the person at G would be too near the partition CN, unless the mirror F was extremely large.

The effect of this and every similar deception is greatly increased when the persons are illuminated with a strong light, and the rest of the apartment as dark as possible; but whatever precautions are taken, and however skilfully plane mirrors are combined, it is not easy to produce with them any very successful illusions.

The concave mirror is the staple instrument of the magician’s cabinet, and must always perform a principal part in all optical combinations. In order to be quite perfect, every concave mirror should have its surface elliptical, so that if any object is placed in one focus of the ellipse, an inverted image of it will be formed in the other focus. This image, to a spectator rightly placed, appears suspended in the air, so that if the mirror and the object are hid from his view, the effect must appear to him almost supernatural.

The method of exhibiting the effect of concave mirrors most advantageously is shown in Fig. 4, where CD is the partition of a room having in it a square opening EF, the centre of which is about five feet above the floor. This opening might be surrounded with a picture-frame, and a painting which exactly filled it might be so connected with a pulley that it could be either slipped aside, or raised so as to leave the frame empty. A large concave mirror MN is then placed in another apartment, so that when any object is placed at A, a distinct image of it may be formed in the62 centre of the opening EF. Let us suppose this object to be a plaster cast of any object made as white as possible, and placed in an inverted position at A. A strong light should then be thrown upon it by a powerful lamp, the rays of which are prevented from reaching the opening EF. When this is done, a spectator placed at O will see an erect image of the statue at B, the centre of the opening, standing in the air, and differing from the real statue only in being a little larger, while the apparition will be wholly invisible to other spectators placed at a little distance on each side of him.

Fig. 4.

If the opening EF is filled with smoke, rising either from a chafing-dish, in which incense is burnt, or made to issue in clouds from some opening below, the image will appear in the middle of the smoke depicted upon it as upon a ground, and capable of being seen by those spectators who could not see the image of the air. The rays of light, in place of proceeding without63 obstruction to an eye at O, are reflected as it were from those minute particles of which the smoke is composed, in the same manner as a beam of light is rendered more visible by passing through an apartment filled with dust or smoke.

It has long been a favourite experiment to place at A a white and strongly illuminated human skull, and to exhibit an image of it amid the smoke of a chafing-dish at B; but a more terrific effect would be produced if a small skeleton suspended by invisible wires were placed as an object at A. Its image suspended in the air at B, or painted upon smoke, could not fail to astonish the spectator.

The difficulty of placing a living person in an inverted position, as an object at A, has no doubt prevented the optical conjuror from availing himself of so admirable a resource; but this difficulty may be removed by employing a second concave mirror. The second mirror may be so placed as to reflect towards MN the rays proceeding from an erect living object, and to form an inverted image of this object at A. An erect image of this inverted image will then be formed at B, either suspended in the air, or depicted upon a wreath of smoke. This aërial image will exhibit the precise form and colours and movements of the living object, and it will maintain its character as an apparition if any attempt is made by the spectator to grasp its unsubstantial fabric.

A deception of an alarming kind, called the Mysterious dagger, has been long a favourite exhibition. If a person with a drawn and highly polished dagger, illuminated by a strong light,64 stands a little farther from a concave mirror than its principal focus, he will perceive in the air between himself and the mirror an inverted and diminished image of his own person, with the dagger similarly brandished. If he aims the dagger at the centre of the mirrors concavity, the two daggers will meet point to point, and, by pushing it still farther from him towards the mirror, the imaginary dagger will strike at his heart. In this case it is necessary that the direction of the real dagger coincides with a diameter of the sphere of which the mirror is a part; but if its direction is on one side of that diameter, the direction of the imaginary dagger will be as far on the other side of the diameter, and the latter will aim a blow at any person who is placed in the proper position for receiving it. If the person who bears the real dagger is therefore placed behind a screen, or otherwise concealed from the view of the spectator, who is made to approach to the place of the image, the thrust of the polished steel at his breast will not fail to produce a powerful impression. The effect of this experiment would no doubt be increased by covering with black cloth the person who holds the dagger, so that the image of his hand only should be seen, as the inverted picture of him would take away from the reality of the appearance. By using two mirrors, indeed, this defect might be remedied, and the spectator would witness an exact image of the assassin aiming the dagger at his life.

The common way of making this experiment is to place a basket of fruit above the dagger, so that a distinct aërial image of the fruit is formed65 in the focus of the mirror. The spectator, having been desired to take some fruit from the basket, approaches for that purpose, while a person properly concealed withdraws the real basket of fruit with one hand, and with the other advances the dagger, the image of which being no longer covered by the fruit, strikes at the body of the astonished spectator.

The powers of the concave mirror have been likewise displayed in exhibiting the apparition of an absent or deceased friend. For this purpose, a strongly illuminated bust or picture of the person is placed before the concave mirror, and a distinct image of the picture will be seen either in the air or among smoke, in the manner already described. If the background of the picture is temporarily covered with lamp-black, so that there is no light about the picture but what falls upon the figure, the effect will be more complete.

As in all experiments with concave mirrors, the size of the aërial image is to that of the real object as their distances from the mirror, we may, by varying the distance of the object, increase or diminish the size of the image. In doing this, however, the distance of the image from the mirror is at the same time changed, so that it would quit the place most suitable for its exhibition. This defect may be removed by simultaneously changing the place both of the mirror and the object, so that the image may remain stationary, expanding itself from a luminous spot to a gigantic size, and again passing through all intermediate magnitudes, till it vanishes in a cloud of light.

Those who have studied the effects of concave66 mirrors of a small size, and without the precautions necessary to ensure deception, cannot form any idea of the magical effect produced by this class of optical apparitions. When the instruments of illusion are themselves concealed,—when all extraneous lights but those which illuminate the real object are excluded,—when the mirrors are large and well-polished, and truly formed,—the effect of the representation on ignorant minds is altogether overpowering; while even those who know the deception, and perfectly understand its principles, are not a little surprised at its effects. The inferiority in the effects of a common concave mirror to that of a well-arranged exhibition is greater even than that of a perspective picture, hanging in an apartment, to the same picture exhibited under all the imposing accompaniments of a dioramic representation.

It can scarcely be doubted, that a concave mirror was the principal instrument by which the heathen gods were made to appear in the ancient temples. In the imperfect accounts which have reached us of these apparitions, we can trace all the elements of an optical illusion. In the ancient temple of Hercules at Tyre, Pliny mentions that there was a seat made of a consecrated stone, “from which the gods easily rose.” Esculapius often exhibited himself to his worshippers in his temple at Tarsus; and the temple of Enguinum in Sicily was celebrated as the place where the goddesses exhibited themselves to mortals. Iamblichus actually informs us, that the ancient magicians caused the gods to appear among the vapours disengaged from fire; and when the conjuror Maximus terrified his audience67 by making the statue of Hecate laugh, while in the middle of the smoke of burning incense, he was obviously dealing with the image of a living object dressed in the costume of the sorceress.

The character of these exhibitions in the ancient temples is so admirably depicted in the following passage of Damascius, quoted by M. Salverte, that we recognise all the optical effects which have been already described. “In a manifestation,” says he, “which ought not to be revealed ... there appeared on the wall of the temple a mass of light, which at first seemed to be very remote; it transformed itself, in coming nearer, into a face evidently divine and supernatural, of a severe aspect, but mixed with gentleness, and extremely beautiful. According to the institutions of a mysterious religion, the Alexandrians honoured it as Osiris and Adonis.”

Among more modern examples of this illusion, we may mention the case of the Emperor Basil of Macedonia. Inconsolable at the loss of his son, this sovereign had recourse to the prayers of the Pontiff Theodore Santabaren, who was celebrated for his power of working miracles. The ecclesiastical conjuror exhibited to him the image of his beloved son, magnificently dressed and mounted upon a superb charger: the youth rushed towards his father, threw himself into his arms, and disappeared. M. Salverte judiciously observes, that this deception could not have been performed by a real person who imitated the figure of the young prince. The existence of this person, betrayed by so remarkable a resemblance, and by the trick of the exhibition, could not fail to have been discovered and denounced,68 even if we could explain how the son could be so instantaneously disentangled from his father’s embrace. The emperor, in short, saw the aërial image of a picture of his son on horseback; and as the picture was brought nearer the mirror, the image advanced into his arms, when it of course eluded his affectionate grasp.

These and other allusions to the operations of the ancient magic, though sufficiently indicative of the methods which were employed, are too meagre to convey any idea of the splendid and imposing exhibitions which must have been displayed. A national system of deception, intended as an instrument of government, must have brought into requisition not merely the scientific skill of the age, but a variety of subsidiary contrivances calculated to astonish the beholder, to confound his judgment, to dazzle his senses, and to give a predominant influence to the peculiar imposture which it was thought desirable to establish. The grandeur of the means may be inferred from their efficacy, and from the extent of their influence.

This defect, however, is, to a certain degree, supplied by an account of a modern necromancy, which has been left us by the celebrated Benvenuto Cellini, and in which he himself performed an active part.

“It happened,” says he, “through a variety of odd accidents, that I made acquaintance with a Sicilian priest, who was a man of genius, and well versed in the Latin and Greek authors. Happening one day to have some conversation with him when the subject turned upon the art of necromancy, I, who had a great desire to know69 something of the matter, told him, that I had all my life felt a curiosity to be acquainted with the mysteries of this art.

“The priest made answer, ‘that the man must be of a resolute and steady temper who enters upon that study.’ I replied, ‘that I had fortitude and resolution enough, if I could but find an opportunity.’ The priest subjoined, ‘If you think you have the heart to venture, I will give you all the satisfaction you can desire.’ Thus we agreed to enter upon a plan of necromancy. The priest one evening prepared to satisfy me, and desired me to look out for a companion or two. I invited one Vincenzio Romoli, who was my intimate acquaintance: he brought with him a native of Pistoia, who cultivated the black art himself. We repaired to the Colosseo, and the priest, according to the custom of necromancers, began to draw circles upon the ground, with the most impressive ceremonies imaginable: he likewise brought hither asafœtida, several precious perfumes, and fire, with some compositions also, which diffused noisome odours. As soon as he was in readiness, he made an opening to the circle, and having taken us by the hand, ordered the other necromancer, his partner, to throw the perfumes into the fire at a proper time, entrusting the care of the fire and perfumes to the rest; and thus he began his incantations. This ceremony lasted above an hour and a half, when there appeared several legions of devils, insomuch that the amphitheatre was quite filled with them. I was busy about the perfumes, when the priest, perceiving there was a considerable number of infernal spirits, turned to me and said,70 ‘Benvenuto, ask them something.’ I answered, ‘Let them bring me into the company of my Sicilian mistress Angelica.’ That night he obtained no answer of any sort; but I had received great satisfaction in having my curiosity so far indulged. The necromancer told me it was requisite we should go a second time, assuring me that I should be satisfied in whatever I asked; but that I must bring with me a pure, immaculate boy.

“I took with me a youth who was in my service, of about twelve years of age, together with the same Vincenzio Romoli, who had been my companion the first time, and one Agnolino Gaddi, an intimate acquaintance, whom I likewise prevailed on to assist at the ceremony. When we came to the place appointed, the priest having made his preparations as before, with the same and even more striking ceremonies, placed us within the circle, which he had likewise drawn with a more wonderful art and in a more solemn manner than at our former meeting. Thus, having committed the care of the perfumes and the fire to my friend Vincenzio, who was assisted by Agnolino Gaddi, he put into my hand a pintaculo or magical chart, and bid me turn it towards the places that he should direct me; and under the pintaculo I held the boy. The necromancer, having begun to make his tremendous invocations, called by their names a multitude of demons who were the leaders of the several legions, and questioned them, by the power of the eternal uncreated God, who lives for ever, in the Hebrew language, as likewise in Latin and Greek; insomuch that the amphitheatre was71 almost in an instant filled with demons more numerous than at the former conjuration. Vincenzio Romoli was busied in making a fire, with the assistance of Agnolino, and burning a great quantity of precious perfumes. I, by the directions of the necromancer, again desired to be in the company of my Angelica. The former thereupon turning to me said,—’Know, they have declared that, in the space of a month, you shall be in her company.’

“He thus requested me to stand resolutely by him, because the legions were now above a thousand more in number than he had designed; and besides, these were the most dangerous; so that, after they had answered my question, it behoved him to be civil to them, and dismiss them quietly. At the same time the boy under the pintaculo was in a terrible fright, saying, that there were in that place a million of fierce men, who threatened to destroy us; and that, moreover, four armed giants of enormous stature were endeavouring to break into our circle. During this time, whilst the necromancer, trembling with fear, endeavoured by mild and gentle methods to dismiss them in the best way he could, Vincenzio Romoli, who quivered like an aspen leaf, took care of the perfumes. Though I was as much terrified as any of them, I did my utmost to conceal the terror I felt; so that I greatly contributed to inspire the rest with resolution; but the truth is, I gave myself over for a dead man, seeing the horrid fright the necromancer was in. The boy placed his head between his knees and said, ‘In this posture will I die; for we shall all surely perish.’ I told him that all these demons72 were under us, and what he saw was smoke and shadow; so bid him hold up his head and take courage. No sooner did he look up than he cried out, ‘The whole amphitheatre is burning, and the fire is just falling upon us.’ So covering his eyes with his hands, he again exclaimed, ‘that destruction was inevitable, and desired to see no more.’ The necromancer entreated me to have a good heart, and take care to burn proper perfumes; upon which I turned to Romoli, and bid him burn all the most precious perfumes he had. At the same time I cast my eye upon Agnolino Gaddi, who was terrified to such a degree that he could scarce distinguish objects, and seemed to be half dead. Seeing him in this condition, I said, ‘Agnolino, upon these occasions a man should not yield to fear, but should stir about and give his assistance, so come directly and put on some more of these.’ The effects of poor Agnolino’s fear were overpowering. The boy, hearing a crepitation, ventured once more to raise his head, when, seeing me laugh, he began to take courage, and said ‘that the devils were flying away with a vengeance.’

“In this condition we stayed, till the bell rang for morning prayers. The boy again told us, that there remained but few devils, and these were at a great distance. When the magician had performed the rest of his ceremonies, he stripped off his gown, and took up a wallet full of books which he had brought with him.

“We all went out of the circle together, keeping as close to each other as we possibly could, especially the boy, who had placed himself in the middle, holding the necromancer by the coat,73 and me by the cloak. As we were going to our houses in the quarter of Banchi, the boy told us that two of the demons whom we had seen at the amphitheatre went on before us leaping and skipping, sometimes running upon the roofs of the houses, and sometimes upon the ground. The priest declared, that though he had often entered magic circles, nothing so extraordinary had ever happened to him. As we went along, he would fain persuade me to assist with him at consecrating a brook, from which, he said, we should derive immense riches; we should then ask the demons to discover to us the various treasures with which the earth abounds, which would raise us to opulence and power; but that these love-affairs were mere follies, from whence no good could be expected. I answered, ‘that I would readily have accepted his proposal, if I understood Latin.’ He redoubled his persuasions, assuring me, that the knowledge of the Latin language was by no means material. He added, that he could have Latin scholars enough, if he had thought it worth while to look out for them, but that he could never have met with a partner of resolution and intrepidity equal to mine, and that I should by all means follow his advice. Whilst we were engaged in this conversation we arrived at our respective houses, and all that night dreamt of nothing but devils.”

It is impossible to peruse the preceding description without being satisfied that the legions of devils were not produced by any influence upon the imaginations of the spectators, but were actual optical phantasms, or the images of pictures or objects produced by one or more concave74 mirrors or lenses. A fire is lighted, and perfumes and incense are burnt, in order to create a ground for the images, and the beholders are rigidly confined within the pale of the magic circle. The concave mirror and the objects presented to it having been so placed that the persons within the circle could not see the aërial image of the objects by the rays deeply reflected from the mirror, the work of deception was ready to begin. The attendance of the magician upon his mirror was by no means necessary. He took his place along with the spectators within the magic circle. The images of the devils were all distinctly formed in the air immediately above the fire, but none of them could be seen by those within the circle. The moment, however, that perfumes were thrown into the fire to produce smoke, the first wreath of smoke that rose through the place of one or more of the images, would reflect them to the eyes of the spectator, and they could again disappear if the wreath was not followed by another. More and more images would be rendered visible as new wreaths of smoke arose, and the whole group would appear at once when the smoke was uniformly diffused over the place occupied by the images.

The “compositions which diffused noisome odours” were intended to intoxicate or stupify the spectators, so as to increase their liability to deception, or to add to the real phantasms which were before their eyes, others which were the offspring only of their own imaginations. It is not easy to gather from the description what parts of the exhibition were actually presented to the eyes of the spectators, and what parts of it75 were imagined by themselves. It is quite evident that the boy, as well as Agnolino Gaddi, were so overpowered with terror that they fancied many things which they did not see; but when the boy declares that four armed giants, of an enormous stature, were threatening to break into the circle, he gives an accurate description of the effect that would be produced by pushing the figures nearer the mirror, and then magnifying their images, and causing them to advance towards the circle. Although Cellini declares that he was trembling with fear, yet it is quite evident that he was not entirely ignorant of the machinery which was at work; for in order to encourage the boy, who was almost dead with fear, he assured them that the devils were under their power, and that “what he saw was smoke and shadow.”

Mr. Roscoe, from whose Life of Cellini the preceding description is taken, draws a similar conclusion from the consolatory words addressed to the boy, and states that they “confirm him in the belief, that the whole of these appearances, like a phantasmagoria, were merely the effects of a magic lantern produced on volumes of smoke from various kinds of burning wood.” In drawing this conclusion, Mr. Roscoe has not adverted to the fact, that this exhibition took place about the middle of the 16th century, while the magic lantern was not invented by Kircher till towards the middle of the 17th century; Cellini having died in 1570, and Kircher having been born in 1601. There is no doubt that the effects described could be produced by this instrument, but we are not entitled to have recourse to any other means76 of explanation but those which were known to exist at the time of Cellini. If we suppose, however, that the necromancer either had a regular magic lantern, or that he had fitted up his concave mirror in a box containing the figures of his devils, and that this box with its lights was carried home with the party, we can easily account for the declaration of the boy, “that as they were going home to their houses in the quarter of Banchi, two of the demons whom we had seen at the amphitheatre went on before us leaping and skipping, sometimes running upon the roofs of the houses, and sometimes upon the ground.”

The introduction of the magic lantern as an optical instrument supplied the magicians of the 17th century with one of their most valuable tools. The use of the concave mirror, which does not appear to have been even put up into the form of an instrument, required a separate apartment, or at least that degree of concealment which it was difficult on ordinary occasions to command; but the magic lantern, containing in a small compass its lamp, its lenses, and its sliding figures, was peculiarly fitted for the itinerant conjuror, who had neither the means of providing a less portable and more extensive apparatus, nor the power of transporting and erecting it.

The magic lantern shown in the annexed figure consists of a dark lantern, AB, containing a lamp G, and a concave metallic mirror, MN, and it is so constructed that when the lamp is lighted not a ray of light is able to escape from it. Into the side of the lantern is fitted a double tube, CD, the outer half of which D is capable of moving within the other half. A large plano-convex lens77 C, is fixed at the inner end of the double tube, and a small convex lens D, at the outer end; and to the fixed tube CE, there is joined a groove EF, in which the sliders containing the painted objects are placed, and through which they can be moved. Each slider contains a series of figures or pictures painted on glass with highly transparent colours. The direct light of the lamp G, and the light reflected from the mirror MN, falling upon the illuminating lens C, is concentrated by it so as to throw a brilliant light upon the painting on the slider, and as this painting is in the conjugate focus of the convex lens D, a magnified image of it will be formed on a white wall or white cloth placed at PQ. If the lens D is brought nearer to EF, or to the picture, the distinct image will be more magnified, and will be formed at a greater distance from D, so that if there is any particular distance of the image which is more convenient than another, or any particular size of the object which we wish, it can78 be obtained by varying the distance of the lens D from EF.

Fig. 5.

When the image is received on an opaque ground, as is commonly the case, the spectators are placed in the same room with the lantern; but, for the purposes of deception, it would be necessary to place the lantern in another apartment like the mirror in Fig. 4, and to throw the magnified pictures on a large plate of ground glass, or a transparent gauze screen, stretched across an opening EF, Fig. 4, made in the partition which separates the spectators from the exhibitor. The images might, like those of the concave mirror, be received upon wreaths of smoke. These images are of course always inverted in reference to the position of the painted objects; but in order to render them really erect, we have only to invert the sliders. The representations of the magic lantern never fail to excite a high degree of interest, even when exhibited with the ordinary apparatus; but by using double sliders, and varying their movements, very striking effects may be produced. A smith, for example, is made to hammer upon his anvil,—a figure is thrown into the attitude of terror by the introduction of a spectral apparition, and a tempest at sea is imitated, by having the sea on one slider, and the ships on other sliders, to which an undulatory motion is communicated.

The magic lantern is susceptible of great improvement in the painting of the figures, and in the mechanism and combination of the sliders. A painted figure, which appears well executed to the unassisted eye, becomes a mere daub when79 magnified 50 or 100 times; and when we consider what kind of artists are employed in their execution, we need not wonder that this optical instrument has degenerated into a mere toy for the amusement of the young. Unless for public exhibition, the expense of exceedingly minute and spirited drawings could not be afforded; but I have no doubt that if such drawings were executed, a great part of the expense might be saved by engraving them on wood, and transferring their outline to the glass sliders.

A series of curious representations might be effected, by inserting glass plates containing suitable figures in a trough having two of its sides parallel, and made of plate glass. The trough must be introduced at EF, so that the figure on the glass is at the proper distance from the object lens D. When the trough is filled with water, or with any transparent fluid, the picture at PQ will be seen with the same distinctness as if the figure had been introduced by itself into the groove EF; but if any transparent fluid of a different density from water is mixed with it, so as to combine with it quickly or slowly, the appearance of the figure displayed at PQ will undergo singular changes. If spirits of wine, or any ardent spirit, are mixed with the water, so as to produce throughout its mass partial variations of density, the figure at PQ, will be as it were broken down into a thousand parts, and will recover its continuity and distinctness when the two fluids have combined. If a fluid of less density than water is laid gently upon the water, so as to mix with it gradually, and produce a regular diminution of density downwards—or if80 saline substances, soluble in water, are laid at the bottom of the trough, the density will diminish upwards, and the figure will undergo the most curious elongations and contractions. Analogous effects may be produced by the application of heat to the surface or sides of the trough, so that we may effect at the same time both an increase and a diminution in the density of the water, in consequence of which the magnified images will undergo the most remarkable transformations. It is not necessary to place the glass plate which contains the figure within the trough. It may be placed in front of it, and by thus creating as it were an atmosphere with local variations of density, we may exhibit the phenomena of the mirage and of looming, in which the inverted images of ships and other objects are seen in the air, as described in another letter.

The power of the magic lantern has been greatly extended by placing it on one side of the transparent screen of taffeta which receives the images, while the spectators are placed on the other side, and by making every part of the glass sliders opaque, excepting the part which forms the figures. Hence all the figures appear luminous on a black ground, and produce a much greater effect with the same degree of illumination. An exhibition depending on these principles was brought out by M. Philipstall in 1802, under the name of the Phantasmagoria, and when it was shown in London and Edinburgh, it produced the most impressive effects upon the spectators. The small theatre of exhibition was lighted only by one hanging lamp, the flame of which was drawn up into an opaque chimney or81 shade when the performance began. In this “darkness visible” the curtain rose and displayed a cave with skeletons and other terrific figures in relief upon its walls. The flickering light was then drawn up beneath its shroud, and the spectators in total darkness found themselves in the middle of thunder and lightning. A thin transparent screen had, unknown to the spectators, been let down after the disappearance of the light, and upon it the flashes of lightning and all the subsequent appearances were represented. This screen being half-way between the spectators and the cave which was first shown, and being itself invisible, prevented the observers from having any idea of the real distance of the figures, and gave them the entire character of aërial pictures. The thunder and lightning were followed by the figures of ghosts, skeletons, and known individuals, whose eyes and mouth were made to move by the shifting of combined sliders. After the first figure had been exhibited for a short time, it began to grow less and less, as if removed to a great distance, and at last vanished in a small cloud of light. Out of this same cloud the germ of another figure began to appear, and gradually grew larger and larger, and approached the spectators, till it attained its perfect development. In this manner the head of Dr. Franklin was transformed into a skull; figures which retired with the freshness of life came back in the form of skeletons, and the retiring skeletons returned in the drapery of flesh and blood.

The exhibition of these transmutations was followed by spectres, skeletons, and terrific figures, which, instead of receding and vanishing as82 before, suddenly advanced upon the spectators, becoming larger as they approached them, and finally vanished by appearing to sink into the ground. The effect of this part of the exhibition was naturally the most impressive. The spectators were not only surprised but agitated, and many of them were of opinion that they could have touched the figures. M. Robertson, at Paris, introduced along with his pictures the direct shadows of living objects, which imitated coarsely the appearance of those objects in a dark night or in moonlight.

All these phenomena were produced by varying the distance of the magic lantern AB, Fig 5, from the screen PQ, which remained fixed, and at the same time keeping the image upon the screen distinct, by increasing the distance of the lens D from the sliders in EF. When the lantern approached to PQ, the circle of light PQ, or the section of the cone of rays PDQ, gradually diminished, and resembled a small bright cloud, when D was close to the screen. At this time a new figure was put in, so that when the lantern receded from the screen, the old figure seemed to have been transformed into the new one. Although the figure was always at the same distance from the spectators, yet, owing to its gradual diminution in size, it necessarily appeared to be retiring to a distance. When the magic lantern was withdrawn from PQ, and the lens D at the same time brought nearer to EF, the image in PQ gradually increased in size, and therefore seemed in the same proportion to be approaching the spectators.

Superior as this exhibition was to any repre83sentation that had been previously made by the magic lantern, it still laboured under several imperfections. The figures were poorly drawn, and in other respects not well executed, and no attempt whatever was made to remove the optical incongruity of the figures becoming more luminous when they retired from the observer, and more obscure when they approached to him. The variation of the distance of the lens D from the sliders in EF was not exactly adapted to the motion of the lantern to and from the screen, so that the outline of the figures was not equally distinct during their variations of magnitude.

Dr. Thomas Young suggested the arrangement shown in Fig. 6 for exhibiting the phantasmagoria.

Fig. 6.

The magic lantern is mounted on a small car H, which runs on wheels WW. The direct light of the lamp G, and that reflected from the mirror M, is condensed by the illuminating lenses CC, upon the transparent figures in the opaque sliders at E, and the image of these figures is formed at PQ, by the object lens D. When the84 car H is drawn back on its wheels, the rod IK brings down the point K, and by means of the rod KL, pushes the lens D nearer to the sliders in EF, and when the car advances to PQ, the point K is raised, and the rod KL draws out the lens D from the slider, so that the image is always in the conjugate focus of D, and therefore distinctly painted on the screen. The rod KN must be equal in length to IK, and the point I must be twice the focal length of the lens D before the object, L being immediately under the focus of the lens. In order to diminish the brightness of the image when it grows small and appears remote, Dr. Young contrived that the support of the lens D should suffer a screen S to fall and intercept a part of the light. This method, however, has many disadvantages, and we are satisfied, that the only way of producing a variation in the light corresponding to the variation in the size of the image, is to use a single illuminating lens C, and to cause it to approach EF, and throw less light upon the figures when D is removed from EF, and to make C recede from EF when D approaches to it. The lens C should therefore be placed in a mean position, corresponding to a mean distance of the screen, and to the ordinary size of the figures, and should have the power of being removed from the slider EF, when a greater intensity of light is required for the images when they are rendered gigantic, and of being brought close to EF when the images are made small. The size of the lens C ought of course to be such that the section of its cone of rays at EF is equal to the size of the figure on the slider when C is at its greatest distance from the slider.

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The method recommended by Dr. Young for pulling out and pushing in the object lens D, according as the lantern approaches to or recedes from the screen, is very ingenious and effective. It is, however, clumsy in itself, and the connexion of the levers with the screen, and their interposition between it and the lantern, must interfere with the operations of the exhibitor. It is, besides, suited only to short distances between the screen and the lantern; for when that distance is considerable, as it must sometimes require to be, the levers KL, KI, KT, would bend by the least strain, and become unfitted for their purpose. For these reasons the mechanism which adjusts the lens D should be moved by the axle of the front wheels, the tube which contains the lens should be kept at its greatest distance from EF by a slender spring, and should be pressed to its proper distance by the action of a spiral cam suited to the optical relation between the two conjugate focal distances of the lens.

Superior as the representations of the phantasmagoria are to those of the magic lantern, they are still liable to the defect which we have mentioned, namely, the necessary imperfection of the minute transparent figures when magnified. This defect cannot be remedied by employing the most skilful artists. Even Michael Angelo would have failed in executing a figure an inch long with transparent varnishes, when all its imperfections were to be magnified. In order, therefore, to perfect the art of representing phantasms, the objects must be living ones, and in place of chalky ill-drawn figures, mimicking humanity by the most absurd gesticulations, we shall have phantasms86 of the most perfect delineation, clothed in real drapery, and displaying all the movements of life. The apparatus by which such objects may be used, may be called the catadioptrical phantasmagoria, as it operates both by reflexion and refraction.

Fig. 7.

The combination of mirrors and lenses which seems best adapted for this purpose is shown in Fig. 7, where AB is a living figure placed before a large concave mirror MN, by means of which a diminished and inverted image of it is formed at ab. If PQ is the transparent screen upon which the image is to be shown to the spectators on the right hand of it, a large lens LL must be so placed before the image ab, as to form a distinct and erect picture of it at A´B´ upon the screen. When the image A´B´ is required to be the exact size of AB, the lens LL must magnify the small image ab as much as the mirror MN diminishes the figure AB. The living object AB, the mirror MN, and the lens LL, must all be placed in a moveable car for the purpose of producing the87 variations in the size of the phantasms, and the transformations of one figure into another. The contrivance for adjusting the lens LL, to give a distinct picture at different distances of the screen, will, of course, be required in the present apparatus. In order to give full effect to the phantasms, the living objects at AB will require to be illuminated in the strongest manner, and should always be dressed either in white or in very luminous colours; and, in order to give them relief, a black cloth should be stretched at some distance behind them. Many interesting effects might also be produced by introducing at AB fine paintings and busts.

It would lead us into too wide a field were we to detail the immense variety of resources which the science of optics furnishes for such exhibitions. One of these, however, is too useful to be passed without notice. If we interpose a prism with a small refracting angle between the image ab, Fig. 7, and the lens LL, the part of the figure immediately opposite to the prism will be as it were detached from the figure, and will be exhibited separately on the screen PQ. Let us suppose that this part is the head of the figure. It may be detached vertically, or lifted from the body as if it were cut off, or it may be detached downwards and placed on the breast as if the figure were deformed. In detaching the head vertically or laterally, an opaque screen must be applied to prevent any part of the head from being seen by rays which do not pass through the prism; but this and other practical details will soon occur to those who put the method to an experimental trial. The application of the prism88 is shown in Fig. 8, where ab is the inverted image formed by a concave mirror, ABC a prism with a small refracting angle BCA, placed between ab and the lens LL, s a small opaque screen, and AB the figure with its head detached. A hand may be made to grasp the hair of the head, and the aspect of death may be given to it, as if it had been newly cut off. Such a representation could be easily made, and the effect upon the spectators would be quite overpowering. The lifeless head might then be made to recover its vitality, and be safely replaced upon the figure. If the head A of the living object AB, Fig. 7, is covered with black cloth, the head of a person or of an animal placed above A might be set upon the shoulders of the figure AB by the refraction of a prism.

Fig. 8.

When the figure ab, Fig. 8, is of very small dimensions, as in the magic lantern, a small prism of glass would answer the purpose required of it; but in public exhibitions, where the image ab must be of a considerable size, if formed by a concave mirror, a very large prism would be89 necessary. This, however, though impracticable with solid glass, may be easily obtained by means of two large pieces of plate glass made into a prismatic vessel and filled with water. Two of the glasses of a carriage window would make a prism capable of doubling the whole of the bust of a living person placed as an object at AB, Fig. 7, so that two perfectly similar phantasms might be exhibited. In those cases where the images before the lens LL are small, they may be doubled and even tripled by interposing a well-prepared plate of calcareous spar, that is, crossed by a thin film. These images would possess the singular character of being oppositely coloured, and of changing their distances and their colours, by slight variations in the positions of the plate.8

In order to render the images which are formed by the glass and water prisms as perfect as possible, it would be easy to make them achromatic, and the figures might be multiplied to any extent by using several prisms, having their refracting edges parallel, for the purpose of giving a similarity of position to all the images.

Fig. 9.

Among the instruments of natural magic which were in use at the revival of science, there was one invented by Kircher for exhibiting the mysterious hand-writing on the wall of an apartment, from which the magician and his apparatus were excluded. The annexed figure represents this apparatus as given by Schottus. The apartment in which the spectators are placed is between LL and GH, and there is an open window in the

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side next LL, GH being the inside of the wall opposite to the window. Upon the face of the plane speculum EF are written the words to be introduced, and when a lens LL is placed at such a distance from the speculum, and of such a focal length, that the letters and the place of their representation are in its conjugate foci, a distinct image of the writing will be exhibited on the wall at GH. The letters on the speculum are of course inverted, as seen at EF, and when they are illuminated by the sun’s rays S, as shown in the figure, a distinct image, as Schottus assures us, may be formed at the distance of 500 feet. In this experiment, the speculum is by no means necessary. If the letters are cut out of an opaque card, and illuminated by the light of the sky in the day, or by a lamp during night, their delineation on the wall would be equally distinct. In the daytime it would be necessary to place the letters at one end of a tube or oblong box, and the lens at the other end. As this deception is performed when the spectators are unprepared for any such exhibition, the warning written in luminous letters on the wall, or any word associated with the fate of the individual observer, could not fail to produce a singular effect upon his mind.91 The words might be magnified, diminished, multiplied, coloured, and obliterated, in a cloud of light, from which they might again reappear by the methods already described, as applicable to the magic lantern.

The art of forming aërial representations was a great desideratum among the opticians of the 17th century. Vitellio and others had made many unsuccessful attempts to produce such images, and the speculations of Lord Bacon on the subject are too curious to be withheld from the reader.

“It would be well bolted out,” says he, “whether great refractions may not be made upon reflexions, as well as upon direct beams. For example, take an empty basin, put an angel or what you will into it; then go so far from the basin till you cannot see the angel, because it is not in a right line; then fill the basin with water, and you shall see it out of its place, because of the refraction. To proceed, therefore, put a looking-glass into a basin of water. I suppose you shall not see the image in a right line or at equal angles, but wide. I know not whether this experiment may not be extended, so as you might see the image and not the glass, which, for beauty and strangeness, were a fine proof, for then you should see the image like a spirit in the air. As, for example, if there be a cistern or pool of water, you shall place over against it the picture of the devil, or what you will, so as that you do not see the water. Then put a looking-glass in the water; now if you can see the devil’s picture aside, not seeing the water, it would look like the devil indeed. They have an old tale in Oxford,92 that Friar Bacon walked between two steeples, which was thought to be done by glasses, when he walked upon the ground.”

Fig. 10.

Kircher also devoted himself to the production of such images, and he has given in the annexed figure his method of producing them. At the bottom of a polished cylindrical vessel AB, he placed a figure CD, which we presume must have been highly illuminated from below, and to the spectators who looked into the vessel in an oblique direction there was exhibited an image placed vertically in the air as if it were ascending at the mouth of the vessel. Kircher assures us that he once exhibited in this manner a representation of the Ascension of our Saviour, and that the images were so perfect that the spectators could not be persuaded, till they had attempted to handle them, that they were not real substances. Although Kircher does not mention it, yet it is manifest that the original figure AB must have93 been a deformed or anamorphous drawing, in order to give a reflected image of just proportions. We doubt, indeed, if the representation or the figure was ever exhibited. It is entirely incompatible with the laws of reflexion.

Fig. 11.

Among the ingenious and beautiful deceptions of the 17th century, we must enumerate that of the re-formation of distorted pictures by reflexion from cylindrical and conical mirrors. In these representations, the original image from which a perfect picture is produced is often so completely94 distorted, that the eye cannot trace in it the resemblance to any regular figure, and the greatest degree of wonder is of course excited, whether the original image is concealed or exposed to view. These distorted pictures may be drawn by strict geometrical rules; but I have shown in Fig. 11 a simple and practical method of executing them. Let MN be an accurate cylinder made of tin-plate or of thick pasteboard. Out of the farther side of it cut a small aperture abcd; and out of the nearer side cut a larger one ABCD, the size of the picture to be distorted. Having perforated the outline of the picture with small holes, place it on the opening ABCD, so that its surface may be cylindrical. Let a candle or a bright luminous object, the smaller the better, be placed at S, as far behind the picture ABCD as the eye is afterwards to be placed before it, and the light passing through the small holes will represent on a horizontal plane a distorted image of the picture A´B´C´D´, which, when sketched in outline with a pencil, and shaded or coloured, will be ready for use. If we now substitute a polished cylindrical mirror of the same size in place of MN, then the distorted picture, when laid horizontally at A´B´C´D´, will be restored to its original state when seen by reflexion at ABCD in the polished mirror. It would be an improvement on this method to place at ABCD a thin and flexible plate of transparent mica, having drawn upon it with a sharp point, or painted upon it, the figure required. The projected image of this figure at A´B´C´D´ may then be accurately copied.

The effect of a cylindrical mirror is shown in95 Fig. 12, which is copied from an old one which we have seen in use.

Fig. 12.

The method above described is equally applicable to concave cylindrical mirrors, and to those of a conical form; and it may also be applied to mirrors of variable curvature, which produce different kinds of distortions from different parts of their surfaces.

Fig. 13.

By employing a mirror whose surface has a variable curvature like ABC, Fig. 13, we obtain an instrument for producing an endless variety of caricatures, all of which are characterised by their resemblance to the original. If a figure MN is placed before such a mirror, it will of course appear distorted and caricatured; but even if the figure takes different distances and positions, the variations which the image undergoes are neither sufficiently numerous nor remarkable to afford much amusement. But if the figure MN is very near the mirror, so that new distortions are produced by the different distances of96 its different parts from the mirror, the most singular caricatures may be exhibited. If the figure, for example, bends forwards his head and the upper part of his body, they will swell in size, leaving his lower extremities short and slender. If it draws back the upper part of the body and advances the limbs, the opposite effect will take place. In like manner different sides of the head, the right or the left side of it, the brow or the chin, may be swelled and contracted at pleasure. By stretching out the arms before the body they become like those of an ourang-outang, and by drawing them back they dwindle into half their regular size. All these effects, which depend chiefly on the agility and skill of the performer, may be greatly increased by suitable distortions in his own features and figure. The family likeness, which is of course never lost in all the variety of figures which are thus produced, adds greatly to the interest of the exhibition; and we have seen individuals so annoyed at recognising97 their own likeness in the hideous forms of humanity which were thus delineated, that they could not be brought to contemplate them a second time. If the figure is inanimate, like the small cast of a statue, the effect is very curious, as the swelling and contracting of the parts and the sudden change of expression give a sort of appearance of vitality to the image. The inflexibility of such a figure, however, is unfavourable to its transformation into caricatures.

Interesting as these metamorphoses are, they lose in the simplicity of the experiment much of the wonder which they could not fail to excite if exhibited on a great scale, where the performer is invisible, and where it is practicable to give an aërial representation of the caricatured figures. This may be done by means of the apparatus shown in Fig. 7,9 where we may suppose AB to be the reduced image seen in the reflecting surface ABC, Fig. 13.10 By bringing this image nearer the mirror MM, Fig. 7, a magnified and inverted image of it may be formed at ab, of such a magnitude as to give the last image in PQ the same size as life. Owing to the loss of light by the two reflexions, a very powerful illumination would be requisite for the original figure. If such an exhibition were well got up, the effect of it would be very striking.

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LETTER V.

Miscellaneous optical illusions—Conversion of cameos into intaglios, or elevations into depressions, and the reverse—Explanation of this class of deceptions—Singular effects of illumination with light of one simple colour—Lamps for producing homogeneous yellow light—Methods of increasing the effect of this exhibition—Method of reading the inscription of coins in the dark—Art of deciphering the effaced inscription of coins—Explanation of these singular effects—Apparent motion of the eyes in portraits—Remarkable examples of this—Apparent motion of the features of a portrait, when the eyes are made to move—Remarkable experiment of breathing light and darkness.

In the preceding letter I have given an account of the most important instruments of Natural Magic which depend on optical principles: but there still remain several miscellaneous phenomena on which the stamp of the marvellous is deeply impressed, and the study of which is pregnant with instruction and amusement.

One of the most curious of these is that false perception in vision by which we conceive depressions to be elevations, and elevations depressions, or by which intaglios are converted into cameos, and cameos into intaglios. This curious fact seems to have been first observed at one of the early meetings of the Royal Society of London, when one of the members, in looking at a99 guinea through a compound microscope of new construction, was surprised to see the head upon the coin depressed, while other members could only see it embossed as it really was.

While using telescopes and compound microscopes, Dr. Gmelin of Wurtemburg observed the same fact. The protuberant parts of objects appeared to him depressed, and the depressed parts protuberant: but what perplexed him extremely, this illusion took place at some times and not at others, in some experiments and not in others, and appeared to some eyes and not to others.

After making a great number of experiments, Dr. Gmelin is said to have constantly observed the following effects: Whenever he viewed any object rising upon a plane of any colour whatever, provided it was neither white nor shining, and provided the eye and the optical tube were directly opposite to it, the elevated parts appeared depressed, and the depressed parts elevated. This happened when he was viewing a seal, and as often as he held the tube of the telescope perpendicularly, and applied it in such a manner that its whole surface almost covered the last glass of the tube. The same effect was produced when a compound microscope was used. When the object hung perpendicularly, from a plane, and the tube was supported horizontally and directly opposite to it, the illusion also took place, and the appearance was not altered when the object hung obliquely and even horizontally. Dr. Gmelin is said to have at last discovered a method of preventing this illusion, which was by looking, not towards the centre of the convexity, but at first to the edges of it only, and then gradually taking100 in the whole. “But why these things should so happen, he did not pretend to determine.”

Fig. 14.

The best method of observing this deception is to view the engraved seal of a watch with the eyepiece of an achromatic telescope, or with a compound microscope, or any combination of lenses which inverts the objects that are viewed through it.11 The depression in the seal will immediately appear an elevation, like the wax impression which is taken from it; and though we know it to be hollow, and feel its concavity with the point of our finger, the illusion is so strong that it continues to appear a protuberance. The cause of this will be understood from Fig. 14, where S is the window of the apartment, or the light which illuminates the hollow seal LR, whose shaded side is of course on the same side L with the light. If we now invert the seal, with one or more lenses, so that it may look in the opposite direction, it will appear to the eye as in Fig. 15, with the shaded side L farthest from the window. But as we know that the window is still on our left hand, and that the light falls in the direction RL, and as everybody with its shaded side 101farthest from the light must necessarily be convex or protuberant, we immediately believe that the hollow seal is now a cameo or bas-relief. The proof which the eye thus receives of the seal being raised, overcomes the evidence of its being hollow, derived from our actual knowledge, and from the sense of touch. In this experiment the deception takes place from our knowing the real direction of the light which falls upon the seal; for if the place of the window, with respect to the seal, had been inverted as well as the seal itself, the illusion could not have taken place.

Fig. 15.

Fig. 16.

In order to explain this better, let us suppose the seal LR, Fig. 14, to be illuminated with a candle S, the place of which we can change at pleasure. If we invert LR, it will rise into a cameo, as in Fig. 15; and if we then place another candle S on the other side of it, as in Fig. 16, the hollow seal will be equally illuminated on all sides, and it will sink down into a102 cavity or intaglio. If the two candles do not illuminate the seal equally, or if any accidental circumstance produces a belief that the light is wholly or principally on one side, the mind will entertain a corresponding opinion respecting the state of the seal, regarding it as a hollow if it believes the light to come wholly or principally from the right hand, and as a cameo if it believes the light to come from the left hand.

If we use a small telescope to invert the seal, and if we cover up all the candle but the flame, and arrange the experiment so that the candle may be inverted along with the image, the seal will still retain its concavity, because the shadow is still on the same side with the illuminating body.

If we make the same experiments with the raised impression of the seal taken upon wax, we shall observe the very same phenomena, the seal being depressed when it alone is inverted, and retaining its convexity when the light is inverted along with it.

The illusion, therefore, under our consideration is the result of an operation of our own minds, whereby we judge of the forms of bodies by the knowledge we have acquired of light and shadow. Hence the illusion depends on the accuracy and extent of our knowledge on this subject; and while some persons are under its influence, others are entirely insensible to it. When the seal or hollow cavity is not polished, but ground, and the surface round it of uniform colour and smoothness, almost every person, whether young or old, learned or ignorant, will be subject to the illusion; because the youngest and the most103 careless observers cannot but know that the shadow of a hollow is always on the side next the light, and the shadow of a protuberance on the side opposite to the light; but if the object is the raised impression of a seal upon wax, I have found that, when inverted, it still seemed raised to the three youngest of six persons, while the three eldest were subject to the deception.

Fig. 17.

This illusion may be dissipated by a process of reasoning arising from the introduction of a new circumstance in the experiment. Thus, let RL, Fig. 17, be the inverted seal, which consequently appears raised, and let an opaque and unpolished pin, A, be placed on one side of the seal. Its shadow will be of course opposite the candle as at B. In this case the seal, which had become a cameo by its inversion, will now sink down into a cavity by the introduction of the pin and its shadow; for as the pin and its shadow are inverted, as shown in Fig. 18, while the candle retains its place, the shadow of the pin falling in the direction AB is a stronger proof to the eye that the light is coming from the right hand, than the actual knowledge of the candle being on the left hand, and therefore the cameo necessarily sinks into a cavity, or the shadow is now on the same side as the light. This experiment will104 explain to us why on some occasions an acute observer will elude the deception, while every other person is subject to it. Let us suppose that a particle of dust, or a little bit of wax, capable of giving a shadow, is adhering to the surface of the seal, an ordinary observer will take no notice of this, or if he does, he will probably not make it a subject of consideration, and will therefore see the head on the seal raised into a cameo; but the attentive observer, noticing the little protuberance, and observing that its shadow lies to the left of it, will instantly infer that the light comes in that direction, and will still see the seal hollow.

Fig. 18.

I have already mentioned that in some cases even the sense of touch does not correct the erroneous perception. We of course feel that the part of the hollow on which the finger is placed is actually hollow; but if we look at the other part of the hollow it will still appear raised.

By using two candles yielding different degrees of light, and thus giving an uncertainty to the direction of the light, we may weaken the illusion in any degree we choose, so as to overpower it by touch, or by a process of reasoning.

I have had occasion to observe a series of analogous phenomena arising from the same cause,105 but produced without any instrument for inverting the object. If AB, for example, is a plate of mother-of-pearl, and LR a circular or any other cavity (Fig. 19) ground or turned in it, then if this cavity is illuminated by a candle or a window at S, in place of there being a shadow of the margin L of the hollow next the light, as there would have been had the body been opaque, a quantity of bright refracted light will appear where there would have been a shadow, and the rest of the cavity will be comparatively obscure, as if it were in shade. The necessary consequence of this is, that the cavity will appear as an elevation when seen only by the naked eye, as it is only an elevated surface that could have its most luminous side at L.

Fig. 19.

Similar illusions take place in certain pieces of polished wood, chalcedony, and mother-of-pearl, where the surface is perfectly smooth. This arises from there being at that place a knot or growth, or nodule, of different transparency from the surrounding mass, and the cause of it will be understood from Fig. 20. Let m o be the surface of a mahogany table, m A o B a section of the table, and m n o a section of a knot more transparent than the rest of the mass. Owing to the trans106parency of the thin edge at o, opposite to the candle S, the side o is illuminated, while the rest of the knot is comparatively dark, so that, on the principles already explained, the spot m n o appears to be a hollow in the table. From this cause arises the appearance of dimples in certain plates of chalcedony, called hammered chalcedony, owing to its having the look of being dimpled with a hammer. The surface on which these cavities are seen is a section of small spherical aggregations of siliceous matter, which exhibit the same phenomena as the cavities in wood. Mother-of-pearl presents the very same phenomena, and it is indeed so common in this substance, that it is nearly impossible to find a mother-of-pearl button or counter which seems to have its surface flat, although they are perfectly so when examined by the touch. Owing to the different refraction of the incident light by the different growths of the shell cut in different directions by the artificial surface, like the annual growth of wood in a dressed plank, the surface has necessarily an unequal and undulating appearance.

Fig. 20.

Among the wonders of science there are perhaps none more surprising than the effects pro107duced upon coloured objects by illuminating them with homogeneous light, or light of one colour. The light which emanates from the sun, and by which all the objects of the material world are exhibited to us, is composed of three different colours, red, yellow, and blue, by the mixture of which in different proportions all the various hues of nature may be produced. These three colours, when mixed in the proportion in which they occur in the sun’s rays, compose a purely white light; but if any body on which this white light falls shall absorb, or stop, or detain within its substance any part of any one or more of these simple colours, it will appear to the eye of that colour which arises from the mixture of all the rays which it does not absorb, or of that colour which white light would have if deprived of the colours which are absorbed. Scarlet cloth, for example, absorbs most of the blue rays and many of the yellow, and hence appears red. Yellow cloth absorbs most of the blue and many of the red rays, and therefore appears yellow; and blue cloth absorbs most of the yellow and red rays. If we were to illuminate the scarlet cloth with pure and unmixed yellow light, it would appear yellow, because the scarlet cloth does not absorb all the yellow rays, but reflects some of them; and if we illuminate blue cloth with yellow light, it will appear nearly black, because it absorbs all the yellow light, and reflects almost none of it. But whatever be the nature and colour of the bodies on which the yellow light falls, the light which it reflects must be yellow, for no other light falls upon them, and those which are not capable of reflecting yellow108 light must appear absolutely black, however brilliant be their colour in the light of day.

Fig. 21.

As the methods now discovered of producing yellow light in abundance were not known to the ancient conjurors, nor even to those of later times, they have never availed themselves of this valuable resource. It has been long known that salt thrown into the wick of a flame produces yellow light, but this light is mixed with blue and green rays, and is, besides, so small in quantity, that it illuminates objects only that are in the immediate vicinity of the flame. A method which I have found capable of producing it in abundance is shown in Fig. 21, where AB is a lamp, containing at A a large quantity of alcohol and water, or ardent spirits, which gradually109 descends into a platina or metallic cup D. This cup is strongly heated by a spirit-lamp L, inclosed in a dark lantern, and when the diluted alcohol in D is inflamed, it will burn with a fierce and powerful yellow flame. If the flame should not be perfectly yellow, owing to an excess of alcohol, a proportion of salt thrown into the cup will answer the same purpose as a further dilution of the alcohol.12

A monochromatic lamp for producing yellow light may be constructed most effectually, by employing a portable gas lamp, containing compressed oil gas. If we allow the gas to escape in a copious stream, and set it on fire, it will form an explosive mixture with the atmospheric air, and will no longer burn with a white flame, but will emit a bluish and reddish light. The force of the issuing gas, or any accidental current of air, is capable of blowing out this flame, so that it is necessary to have a contrivance for sustaining it. The method which I used for this purpose is shown in Fig. 22. A small gas tube a b c, arising from the main burner MN of the gas lamp PQ, terminates above the burner, and has a short tube d e, moveable up and down within it, so as to be gas-tight. This tube d e, closed at e, communicates with the hollow ring f g, in the inside of which four apertures are perforated in such a manner as to throw their jets of gas to the apex of a cone, of which f g is the base. When we cause the gas to flow from the burner M, by opening the main cock A, it will rush into the tube a b c d, and issue in small flames at the four holes in the ring f g. The size110 of these flames is regulated by the cock b. The inflammation, therefore, of the ignited gas will be sustained by these four subsidiary flames through which it passes, independent of any agitation of the air, or of the force with which it issues from the burner. On a projecting arm e h, carrying a ring h, I fixed a broad collar, made of coarse cotton wick, which had been previously soaked in a saturated solution of common salt. When the gas was allowed to escape at M, with such force as to produce a long and broad column of an explosive mixture of gas and atmospheric air, the bluish flame occasioned by the explosion passes through the salted collar, and is converted by it into a mass of homogeneous yellow light. This collar will last a long time without any fresh supply of salt, so that the gas lamp will yield a permanent monochromatic yellow flame, which111 will last as long as there is gas in the reservoir. In place of a collar of cotton wick, a hollow cylinder of sponge, with numerous projecting tufts, may be used, or a collar may be similarly constructed with asbestos cloth, and, if thought necessary, it might be supplied with a saline solution from a capillary fountain.

Fig. 22.

Having thus obtained the means of illuminating any apartment with yellow light, let the exhibition be made in a room with furniture of various bright colours, with oil or water-coloured paintings on the wall. The party which is to witness the experiment should be dressed in a diversity of the gayest colours; and the brightest-coloured flowers and highly-coloured drawings should be placed on the tables. The room being at first lighted with ordinary lights, the bright and gay colours of everything that it contains will be finely displayed. If the white lights are now suddenly extinguished, and the yellow lamps lighted, the most appalling metamorphosis will be exhibited. The astonished individuals will no longer be able to recognize each other. All the furniture in the room, and all the objects which it contains, will exhibit only one colour. The flowers will lose their hues. The paintings and drawings will appear as if they were executed in China ink; and the gayest dresses, the brightest scarlets, the purest lilacs, the richest blues, and the most vivid greens, will all be converted into one monotonous yellow. The complexions of the parties, too, will suffer a corresponding change. One pallid, death-like yellow,

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will envelope the young and the old, and the sallow faces will alone escape from the metamorphosis. Each individual derives merriment from the cadaverous appearance of his neighbour, without being sensible that he is himself one of the ghostly assemblage.

If, in the midst of the astonishment which is thus created, the white lights are restored at one end of the room, while the yellow lights are taken to the other end, one side of the dress of every person, namely, that next the white light, will be restored to its original colours, while the other side will retain its yellow hue. One cheek will appear in a state of health and colour, while the other retains the paleness of death; and, as the individuals change their position, they will exhibit the most extraordinary transformations of colour.

If, when all the lights are yellow, beams of white light are transmitted through a number of holes, like those in a sieve, each luminous spot will restore the colour of the dress or furniture upon which it falls, and the nankeen family will appear all mottled over with every variety of tint. If a magic lantern is employed to throw upon the walls or upon the dresses of the company luminous figures of flowers or animals, the dresses will be painted with these figures in the real colour of the dress itself. Those alone who appeared in yellow, and with yellow complexions, will, to a great degree, escape all these singular changes.

If red and blue light could be produced with the same facility and in the same abundance as yellow light, the illumination of the apartment113 with these lights in succession would add to the variety and wonder of the exhibition. The red light might perhaps be procured in sufficient quantity from the nitrate and other salts of strontian; but it would be difficult to obtain a blue flame of sufficient intensity for the suitable illumination of a large room. Brilliant white lights, however, might be used, having for screens glass troughs containing a mass one or two inches thick of a solution of the ammoniacal carbonate of copper. This solution absorbs all the rays of the spectrum but the blue, and the intensity of the blue light thus produced would increase in the same proportion as the white light employed.

Amongst the numerous experiments with which science astonishes and sometimes even strikes terror into the ignorant, there is none more calculated to produce this effect than that of displaying to the eye in absolute darkness the legend or inscription upon a coin. To do this, take a silver coin (I have always used an old one), and after polishing the surface as much as possible, make the parts of it which are raised rough by the action of an acid, the parts not raised, or those which are to be rendered darkest, retaining their polish. If the coin thus prepared is placed upon a mass of red-hot iron, and removed into a dark room, the inscription upon it will become less luminous than the rest, so that it may be distinctly read by the spectator. The mass of red-hot iron should be concealed from the observer’s eye, both for the purpose of rendering the eye fitter for observing the effect, and of removing all doubt that the inscription is really read in the dark, that is, without receiving any light, direct114 or reflected, from any other body. If, in place of polishing the depressed parts and roughening its raised parts, we make the raised parts polished and roughen the depressed parts, the inscription will now be less luminous than the depressed parts, and we shall still be able to read it, from its being as it were written in black letters on a white ground. The first time I made this experiment, without being aware of what would be the result, I used a French shilling of Louis XV., and I was not a little surprised to observe upon its surface, in black letters, the inscription BENEDICTUM SIT NOMEN DEI.

The most surprising form of this experiment is when we use a coin from which the inscription has been either wholly obliterated, or obliterated in such a degree as to be illegible. When such a coin is laid upon the red-hot iron, the letters and figures become oxidated, and the film of oxide radiating more powerfully than the rest of the coin, the illegible inscription may be now distinctly read, to the great surprise of the observer, who had examined the blank surface of the coin previous to its being placed upon the hot iron. The different appearances of the same coin, according as the raised parts are polished or roughened, are shown in Fig. 23 and 24.

In order to explain the cause of these remarkable effects, we must notice a method which has been long known, though never explained, of deciphering the inscriptions on worn-out coins. This is done by merely placing the coin upon a hot iron; an oxidation takes place over the whole surface of the coin, the film of oxide changing its tint with the intensity or continuance of the115 heat. The parts, however, where the letters of the inscription had existed, oxidate at a different rate from the surrounding parts, so that these letters exhibit their shape, and become legible in consequence of the film of oxide which covers them having a different thickness, and therefore reflecting a different tint from that of the adjacent parts. The tints thus developed sometimes pass through many orders of brilliant colours, particularly pink and green, and settle in a bronze, and sometimes a black tint, resting upon the inscription alone. In some cases the tint left on the trace of the letters is so very faint that it can just be seen, and may be entirely removed by a slight rub of the finger.

Fig. 23.

Fig. 24.

When the experiment is often repeated with the same coin, and the oxidations successively removed after each experiment, the film of oxide continues to diminish, and at last ceases to make its appearance. It recovers the property, however, in the course of time. When the coin is put upon the hot iron, and consequently when the oxidation is the greatest, a considerable smoke arises from the coin, and this diminishes like the116 film of oxide by frequent repetition. A coin which had ceased to emit this smoke, smoked slightly after having been exposed twelve hours to the air. I have found, from numerous trials, that it is always the raised parts of the coin, and in modern coins the elevated ledge round the inscription, that become first oxidated. In an English shilling of 1816, this ledge exhibited a brilliant yellow tint before it appeared on any other part of the coin.

If we use a uniform and homogeneous disc of silver that has never been hammered or compressed, its surface will oxidate equally, provided all its parts are equally heated. In the process of converting this disc into a coin, the sunk parts have obviously been most compressed by the prominent parts of the die, and the elevated parts least compressed, the metal being in the latter left as it were in its natural state. The raised letters and figures on a coin have therefore less density than the other parts, and these parts oxidate sooner or at a lower temperature. When the letters of the legend are worn off by friction, the parts immediately below them have also less density than the surrounding metal, and the site as it were of the letters therefore receives from heat a degree of oxidation, and a colour different from that of the surrounding surface. Hence we obtain an explanation of the revival of the invisible letters by oxidation.

The same influence of difference of density may be observed in the beautiful oxidations which are produced on the surface of highly-polished steel, heated in contact with air, at temperatures between 430° and 630° of Fahren117heit.13 When the steel has hard portions called pins by the workmen, the uniform tint of the film of oxide stops near these hard portions, which always exhibit colours different from those of the rest of the mass. These parts, on account of their increased density, absorb the oxygen of atmospheric air less copiously than the surrounding portions. Hence we see the cause why steel expanded by heat absorbs oxygen, which when united with the metal, forms the coloured superficial film. As the heat increases, a greater quantity of oxygen is absorbed, and the film increases in thickness.

These observations enable us to explain the legibility of inscriptions in the dark, whether the coin is in a perfect state, or the letters of it worn off. All black or rough surfaces radiate light more copiously than polished or smooth surfaces, and hence the inscription is luminous when it is rough, and obscure when it is polished, and the letters covered with black oxide are more luminous than the adjacent parts, on account of the superior radiation of light by the black oxide which covers them.

By the means now described, invisible writing might be conveyed by impressing it upon a metallic surface, and afterwards erasing it by grinding and polishing that surface perfectly smooth. When exposed to a proper degree of heat, the secret would display itself written in oxidated letters. Many amusing experiments might be made upon the same principle.

A series of curious and sometimes alarming118 deceptions, arises from the representation of objects in perspective upon a plane surface. One of the most interesting of these depends on the principles which regulate the apparent direction of the eyes in a portrait. Dr. Wollaston has thought this subject of sufficient importance to be treated at some length in the Philosophical Transactions. When we look at any person we direct to them both our face and our eyes, and in this position the circular iris will be in the middle of the white of the eye ball, or, what is the same thing, there will be the same quantity of white on each side of the iris. If the eyes are now moved to either side, while the head remains fixed, we shall readily judge of the change of their direction by the greater or less quantity of white on each side of the iris. This test, however, accurate as it is, enables us only to estimate the extent to which the eyes deviate in direction from the direction of the face to which they belong. But their direction in reference to the person who views them is entirely a different matter; and Dr. Wollaston is of opinion, that we are not guided by the eyes alone, but are unconsciously aided by the concurrent position of the entire face.

Fig. 26.

If a skilful painter draws a pair of eyes with great correctness directed to the spectator, and deviating from the general position of the face as much as is usual in good portraits, it is very difficult to determine their direction, and they will appear to have different directions to different persons. But what is very curious, Dr. Wollaston has shown that the same pair of eyes may be made to direct themselves either to or from the spectator by the addition of other features in119 which the position of the face is changed. Thus, in Fig. 25, the pair of eyes are looking intently at the spectator, and the face has a corresponding direction; but when we cover up the face in Fig. 25 with the face in Fig. 26, which looks to the right, the eyes change their direction, and look to the right also. In like manner, eyes drawn originally to look a little to the right or the left of the spectator, may be made to look directly at him by adding suitable features.

Fig. 25.

The nose is obviously the principal feature which produces this change of direction, as it is more subject to change of perspective than any of the other features; but Dr. Wollaston has shown by a very accurate experiment, that even a small portion of the nose introduced with the features will carry the eyes along with it. He obtained four exact copies of the same pair of eyes looking at the spectator, by transferring them upon copper from a steel plate, and having added120 to each of two pair of them a nose, in one case directed to the right, and in the other to the left, and to each of the other two pair a very small portion of the upper part of the nose, all the four pair of eyes lost their front direction, and looked to the right or to the left, according to the direction of the nose, or of the portion of it which was added.

But the effect thus produced is not limited, as Dr. Wollaston remarks, to the mere change in the direction of the eyes, “for a total difference of character may be given to the same eyes by a due representation of the other features. A lost look of devout abstraction in an uplifted countenance, may be exchanged for an appearance of inquisitive archness in the leer of a younger face turned downwards and obliquely towards the opposite side,” as in Fig. 27, 28. This, however, is perhaps not an exact expression of the fact. The new character which is said to be given to the eyes is given only to the eyes in combination with the new features, or, what is probably more correct, the inquisitive archness is in the other features, and the eye does not belie it.

Dr. Wollaston has not noticed the converse of these illusions, in which a change of direction is given to fixed features by a change in the direction of the eyes. This effect is finely seen in some magic lantern sliders, where a pair of eyes is made to move in the head of a figure, which invariably follows the motion of the eyeballs.

Fig. 28.

Fig. 27.

Having thus determined the influence which the general perspective of the face has upon the apparent direction of the eyes in a portrait, Dr. Wollaston applies it to the explanation of the121 well-known fact, that when the eyes of a portrait look at a spectator in front of it they will follow him, and appear to look at him in every other direction. This curious fact, which has received less consideration than it merits, has been often skilfully employed by the novelist, in alarming the fears or exciting the courage of his hero. On returning to the hall of his ancestors, his attention is powerfully fixed on the grim portraits which surround him. The parts which they have respectively performed in the family history rise to his mind: his own actions, whether good or evil, are called up in contrast, and as the preserver or the destroyer of his line, he stands, as it were, in judgment before them. His imagination, thus excited by conflicting feelings, transfers a sort of vitality to the canvas, and if the personages do122 not “start from their frames,” they will at least bend upon him their frowns or their approbation. It is in vain that he tries to evade their scrutiny. Wherever he goes their eyes eagerly pursue him; they will seem even to look at him over their shoulders, and he will find it impossible to shun their gaze but by quitting the apartment.

As the spectator in this case changes his position in a horizontal plane, the effect which we have described is accompanied by an apparent diminution in the breadth of the human face, from only seven or eight inches till it disappears at a great obliquity. In moving, therefore, from a front view to the most oblique view of the face, the change in its apparent breadth is so slow that the apparent motion of the head of the figure is scarcely recognized as it follows the spectator. But if the perspective figure has a great breadth in a horizontal plane, such as a soldier firing his musket, an artilleryman his piece of ordnance, a bowman drawing his bow, or a lancer pushing his spear, the apparent breadth of the figure will vary from five to six feet or upwards till it disappears, and therefore the change of apparent magnitude is sufficiently rapid to give the figure the dreaded appearance of turning round, and following the spectator. One of the best examples of this must have been often observed in the foreshortened figure of a dead body lying horizontally, which has the appearance of following the observer with great rapidity, and turning round upon the head as the centre of motion.

The cause of this phenomenon is easily explained. Let us suppose a portrait with its face and its eyes directed straight in front, so as to123 look at the spectator. Let a straight line be drawn through the tip of the nose and half way between the eyes, which we shall call the middle line. On each side of this middle line there will be the same breadth of head, of cheek, of chin, and of neck, and each iris will be in the middle of the white of the eye. If we now go to one side, the apparent horizontal breadth of every part of the head and face will be diminished, but the parts on each side of the middle line will be diminished equally, and at any position, however oblique, there will be the same breadth of face on each side of the middle line, and the iris will be in the centre of the white of the eyeball, so that the portrait preserves all the characters of a figure looking at the spectator, and must necessarily do so wherever he stands.

This explanation might be illustrated by a picture which represents three artillerymen, each firing a piece of ordnance in parallel directions. Let the gun of the middle one be pointed accurately to the eye of the spectator, so that he sees neither its right side nor its left, nor its upper nor its under side, but directly down its muzzle, so that if there was an opening in the breech he would see through it. In like manner the spectator will see the left side of the gun on his left hand, and the right side of the gun on his right hand. If the spectator now changes his place, and takes ever such an oblique position, either laterally or vertically, he must still see the same thing; because nothing else is presented to his view. The gun of the middle soldier must always point to his eye, and the other guns to the right and left of him. They must therefore all three124 seem to move as he moves, and follow his eye in all its changes of place. The same observations are of course applicable to buildings and streets seen in perspective.

In common portraits the apparent motion of the head is generally rendered indistinct by the canvas being imperfectly stretched, as the slightest concavity or convexity entirely deforms the face when the obliquity is considerable. The deception is therefore best seen when the painting is executed on a very flat board, and in colours sufficiently vivid to represent every line in the face with tolerable distinctness at great obliquities. This distinctness of outline is indeed necessary to a satisfactory exhibition of this optical illusion. The most perfect exhibition, indeed, that I ever saw of it was in the case of a painting of a ship upon a sign-board executed in strongly gilt lines. It contained a view of the stern and side of a ship in the stocks, and, owing to the flatness of the board and the brightness of the lines, the gradual development of the figure, from the most violent foreshortening at great obliquities till it attained its perfect form, was an effect which surprised every person that saw it.

Fig. 29.

The only other optical illusion which our limits will permit us to explain, is the very remarkable experiment of what may be truly called breathing light or darkness. Let S be a candle whose light falls at an angle of 56° 45´ upon two glass plates A, B, placed close to each other, and let the reflected rays AC, BD, fall at the same angle upon two similar plates, C, D, but so placed that the plane of reflexion from the latter is at right angles to the plane of reflexion from the former. An125 eye placed at E, and looking at the same time into the two plates C and D, will see very faint images of the candle S, which by a slight adjustment of the plates, may be made to disappear almost wholly allowing the plate C to remain as it is, change the position of D, till its inclination to the ray BD is diminished about 3°, or made nearly 53° 11´. When this is done, the image that had disappeared on looking into D will be restored, so that the spectator at E, upon looking into the two mirrors C, D, will see no light in C, because the candle has nearly disappeared, while the candle is distinctly seen in D. If, while the spectator is looking into these two mirrors, either he or another person breathes upon them gently and quickly, the breath will revive the extinguished image in C, and will extinguish the visible image in D. The following is the cause of this singular result. The light AC, BD, is polarized by reflexion from the plates A, B, because it is incident at the polarizing angle of 56° 45´ for glass. When we breathe upon the plates C, D, we form upon their surface a thin film of water, whose polarizing angle is 53° 11´, so that if the polarized rays AC, BD, fell upon the plates C, D, at an angle of 53° 11´, the126 candle from which they proceeded would not be visible, or they would not suffer reflexion from the plates C, D. At all other angles the light would be reflected and the candles visible. Now the plate D is placed at an angle of 53° 11´ and C at an angle of 56° 45´, so that when a film of water is breathed upon them the light will be reflected from the latter, and none from the former; that is, the act of breathing upon the plates will restore the invisible and extinguish the visible image.

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LETTER VI.

Natural phenomena marked with the marvellous—Spectre of the Brocken described—Analogous phenomena—Aërial spectres seen in Cumberland—Fata Morgana in the Straits of Messina—Objects below the horizon raised and magnified by refraction—Singular example seen at Hastings—Dover Castle seen through the hill on which it stands—Erect and inverted images of distant ships seen in the air—Similar phenomena seen in the Arctic regions—Enchanted coast—Mr. Scoresby recognizes his father’s ship by its aërial image—Images of cows seen in the air—Inverted image of horses seen in South America—Lateral images produced by refraction—Aërial spectres by reflexion—Explanation of the preceding phenomena.

Among the wonders of the natural world which are every day presented to us, without either exciting our surprise or attracting our notice, some are occasionally displayed which possess all the characters of supernatural phenomena. In the names by which they are familiarly known, we recognize the terror which they inspired, and even now, when science has reduced them to the level of natural phenomena, and developed the causes from which they arise, they still retain their primitive importance, and are watched by the philosopher with as intense an interest as when they were deemed the immediate effects of Divine power. Among these phenomena we may enumerate the Spectre of the Brocken, the Fata Morgana of the Straits of Messina, the Spectre Ships which ap128pear in the air, and the other extraordinary effects of the Mirage.14

The Brocken is the name of the loftiest of the Hartz mountains, a picturesque range which lies in the kingdom of Hanover. It is elevated 3,300 feet above the sea, and commands the view of a plain seventy leagues in extent, occupying nearly the two-hundredth part of the whole of Europe, and animated with a population of above five millions of inhabitants. From the earliest periods of authentic history, the Brocken has been the seat of the marvellous. On its summits are still seen huge blocks of granite called the Sorcerer’s Chair and the Altar. A spring of pure water is known by the name of the Magic Fountain, and the Anemone of the Brocken is distinguished by the title of the Sorcerer’s Flower. These names are supposed to have originated in the rites of the great idol Cortho, whom the Saxons worshipped in secret on the summit of the Brocken, when Christianity was extending her benignant sway over the subjacent plains.

As the locality of these idolatrous rites, the Brocken must have been much frequented, and we can scarcely doubt that the spectre which now so often haunts it at sunrise must have been observed from the earliest times; but it is nowhere mentioned that this phenomenon was in any way129 associated with the objects of their idolatrous worship. One of the best accounts of the spectre of the Brocken is that which is given by M. Haue, who saw it on the 23rd of May, 1797. After having been on the summit of the mountain no less than thirty times, he had at last the good fortune of witnessing the object of his curiosity. The sun rose about four o’clock in the morning through a serene atmosphere. In the south-west, towards Achtermannshohe, a brisk west wind carried before it the transparent vapours, which had not yet been condensed into thick heavy clouds. About a quarter past four he went towards the inn, and looked round to see whether the atmosphere would afford him a free prospect towards the south-west, when he observed at a very great distance, towards Achtermannshohe, a human figure of a monstrous size. His hat having been almost carried away by a violent gust of wind, he suddenly raised his hand to his head to protect his hat, and the colossal figure did the same. He immediately made another movement by bending his body,—an action which was repeated by the spectral figure. M. Haue was desirous of making further experiments, but the figure disappeared. He remained, however, in the same position, expecting its return, and in a few minutes it again made its appearance on the Achtermannshohe, when it mimicked his gestures as before. He then called the landlord of the inn, and having both taken the same position which he had before, they looked towards the Achtermannshohe, but saw nothing. In a very short space of time, however, two colossal figures were formed over the above eminence, and after130 bending their bodies and imitating the gestures of the two spectators, they disappeared. Retaining their position, and keeping their eyes still fixed upon the same spot, the two gigantic spectres again stood before them, and were joined by a third. Every movement that they made was imitated by the three figures, but the effect varied in its intensity, being sometimes weak and faint, and at other times strong and well defined.

Fig. 30.

In the year 1798, M. Jordan saw the same phenomenon at sunrise, and under similar circumstances, but with less distinctness, and without any duplication of the figures.15

Phenomena perfectly analogous to the preceding, though seen under less imposing circumstances, have been often witnessed. When the spectator sees his own shadow opposite to the sun upon a mass of thin fleecy vapour passing near him, it not only imitates all his movements, but its head is distinctly encircled with a halo of131 light. The aërial figure is often not larger than life, its size and its apparent distance depending, as we shall afterwards see, upon particular causes. I have often seen a similar shadow when bathing in a bright summer’s day in an extensive pool of deep water. When the fine mud deposited at the bottom of the pool is disturbed by the feet of the bather, so as to be disseminated through the mass of water in the direction of his shadow, his shadow is no longer a shapeless mass formed upon the bottom, but is a regular figure formed upon the floating particles of mud, and having the head surrounded with a halo, not only luminous, but consisting of distinct radiations.

One of the most interesting accounts of aërial spectres with which we are acquainted has been given by Mr. James Clarke, in his Survey of the Lakes of Cumberland, and the accuracy of this account was confirmed by the attestations of two of the persons by whom the phenomena were first seen. On a summer’s evening, in the year 1743, when Daniel Stricket, servant to John Wren, of Wilton Hall, was sitting at the door along with his master, they saw the figure of a man with a dog pursuing some horses along Souterfell-side, a place so extremely steep, that a horse could scarcely travel upon it at all. The figures appeared to run at an amazing pace, till they got out of sight at the lower end of the Fell. On the following morning, Stricket and his master ascended the steep side of the mountain, in the full expectation of finding the man dead, and of picking up some of the shoes of the horses, which they thought must have been cast while galloping at such a furious rate. Their expectations, how132ever, were disappointed. No traces, either of man or horse, could be found, and they could not even discover upon the turf the single mark of a horse’s hoof. These strange appearances seen at the same time by two different persons in perfect health, could not fail to make a deep impression on their minds. They at first concealed what they had seen, but they at length disclosed it, and were laughed at for their credulity.

In the following year, on the 23rd June, 1744, Daniel Stricket, who was then servant to Mr. Lancaster, of Blakehills, (a place near Wilton Hall, and both of which places are only about half a mile from Souterfell,) was walking, about seven o’clock in the evening, a little above the house, when he saw a troop of horsemen riding on Souterfell-side, in pretty close ranks, and at a brisk pace. Recollecting the ridicule that had been cast upon him the preceding year, he continued to observe the figures for some time in silence; but being at last convinced that there could be no deception in the matter, he went to the house, and informed his master that he had something curious to show him. They accordingly went out together; but before Stricket had pointed out the place, Mr. Lancaster’s son had discovered the aërial figures. The family was then summoned to the spot, and the phenomena were seen alike by them all. The equestrian figures seemed to come from the lowest parts of Souterfell, and became visible at a place called Knott. They then advanced in regular troops along the side of the Fell, till they came opposite to Blakehills, when they went over the mountain, after describing a kind of curvilineal path. The133 pace at which the figures moved was a regular swift walk, and they continued to be seen for upwards of two hours, the approach of darkness alone preventing them from being visible. Many troops were seen in succession; and frequently the last but one in a troop quitted his position, galloped to the front, and took up the same pace with the rest. The changes in the figures were seen equally by all the spectators, and the view of them was not confined to the farm of Blakehills only, but they were seen by every person at every cottage within the distance of a mile, the number of persons who saw them amounting to about twenty-six. The attestation of these facts, signed by Lancaster and Stricket, bears the date of the 21st July, 1785.

These extraordinary sights were received not only with distrust, but with absolute incredulity. They were not even honoured with a place in the records of natural phenomena, and the philosophers of the day were neither in possession of analogous facts, nor were they acquainted with those principles of atmospherical refraction upon which they depend. The strange phenomena, indeed, of the Fata Morgana, or the Castles of the Fairy Morgana, had been long before observed, and had been described by Kircher in the 17th century, but they presented nothing so mysterious as the aërial troopers of Souterfell; and the general characters of the two phenomena were so unlike, that even a philosopher might have been excused for ascribing them to different causes.

This singular exhibition has been frequently seen in the straits of Messina, between Sicily and134 the coast of Italy, and whenever it takes place, the people, in a state of exultation, as if it were not only a pleasing but a lucky phenomenon, hurry down to the sea, exclaiming Morgana, Morgana! When the rays of the rising sun form an angle of 45° on the sea of Reggio, and when the surface of the water is perfectly unruffled, either by the wind or the current, a spectator placed upon an eminence in the city, and having his back to the sun and his face to the sea, observes upon the surface of the water superb palaces, with their balconies and windows, lofty towers, herds and flocks grazing in wooded valleys and fertile plains; armies of men on horseback and on foot, with multiplied fragments of buildings, such as columns, pilasters, and arches. These objects pass rapidly in succession along the surface of the sea during the brief period of their appearance. The various objects thus enumerated are pictures of palaces and buildings actually existing on shore, and the living objects are of course only seen when they happen to form a part of the general landscape.

If, at the time that these phenomena are visible, the atmosphere is charged with vapour or dense exhalations, the same objects which are depicted upon the sea will be seen also in the air, occupying a space which extends from the surface to the height of twenty-five feet. These images, however, are less distinctly delineated than the former.

If the air is in such a state as to deposit dew, and is capable of forming the rainbow, the objects will be seen only on the surface of the sea; but they all appear fringed with red, yellow, and135 blue light, as if they were seen through a prism.

In our own country, and in our own times, facts still more extraordinary have been witnessed. From Hastings, on the coast of Sussex, the cliffs on the French coast are fifty miles distant, and they are actually hid by the convexity of the earth; that is, a straight line drawn from Hastings to the French coast would pass through the sea. On Wednesday, the 26th of July, 1798, about five o’clock in the afternoon, Mr. Latham, a Fellow of the Royal Society, then residing at Hastings, was surprised to see a crowd of people running to the sea-side. Upon inquiry into the cause of this, he learned that the coast of France could be seen by the naked eye, and he immediately went down to witness so singular a sight. He distinctly saw the cliffs extending for some leagues along the French coast, and they appeared as if they were only a few miles off. They gradually appeared more and more elevated, and seemed to approach nearer to the eye. The sailors with whom Mr. Latham walked along the waters edge were at first unwilling to believe in the reality of the appearance; but they soon became so thoroughly convinced of it, that they pointed out and named to him the different places which they had been accustomed to visit, and which they conceived to be as near as if they were sailing at a small distance into the harbour. These appearances continued for nearly an hour, the cliffs sometimes appearing brighter and nearer, and at other times fainter and more remote. Mr. Latham then went upon the eastern cliff or hill, which is of considerable height, when,136 as he remarks, a most beautiful scene presented itself to his view. He beheld at once Dungeness, Dover cliffs, and the French coast all along from Calais, Boulogne, &c., to St. Vallery, and, as some of the fishermen affirmed, as far west as Dieppe. With the help of a telescope, the French fishing-boats were plainly seen at anchor, and the different colours of the land upon the heights, together with the buildings, were perfectly discernible. Mr. Latham likewise states that the cape of land called Dungeness, which extends nearly two miles into the sea, and is about sixteen miles in a straight line from Hastings, appeared as if quite close to it, and the vessels and fishing-boats which were sailing between the two places appeared equally near, and were magnified to a high degree. These curious phenomena continued “in the highest splendour” till past eight o’clock, although a black cloud had for some time totally obscured the face of the sun.

A phenomenon no less marvellous was seen by Professor Vince, of Cambridge, and another gentleman, on the 6th of August, 1806, at Ramsgate. The summits v w x y of the four turrets of Dover Castle are usually seen over the hill AB, upon which it stands, lying between Ramsgate and Dover; but on the day above-mentioned, at seven o’clock in the evening, when the air was very still and a little hazy, not only were the tops v w x y of the four towers of Dover Castle seen over the adjacent hill AB, but the whole of the Castle, m n r s, appeared as if it were situated on the side of the hill next Ramsgate, and rising above the hill as much as usual. This phenomenon137 was so very singular and unexpected, that at first sight Dr. Vince thought it an illusion; but upon continuing his observations, he became satisfied that it was a real image of the Castle. Upon this he gave a telescope to a person present, who, upon attentive examination, saw also a very clear image of the Castle as the Doctor had described it. He continued to observe it for about twenty minutes, during which time the appearance remained precisely the same; but rain coming on, they were prevented from making any further observations. Between the observers and the land from which the hill rises there was about six miles of sea, and from thence to the top of the hill there was about the same distance. Their own height above the surface of the water was about seventy feet.

Fig. 31.

This illusion derived great force from the remarkable circumstance, that the hill itself did not appear through the image, as it might have been expected to do. The image of the castle was very strong and well defined, and though the rays from138 the hill behind it must undoubtedly have come to the eye, yet the strength of the image of the castle so far obscured the background, that it made no sensible impression on the observers. Their attention was of course principally directed to the image of the castle; but if the hill behind had been at all visible, Dr. Vince conceives that it could not have escaped their observation, as they continued to look at it for a considerable time with a good telescope.

Fig. 32.

Hitherto our aërial visions have been seen only in their erect and natural positions, either projected against the ground or elevated in the air; but cases have occurred in which both erect and inverted images of objects have been seen in the air, sometimes singly, sometimes combined, sometimes when the real object was invisible, and sometimes when a part of it had begun to show itself to the spectator.

In the year 1793, Mr. Huddart, when residing at Allonby, in Cumberland, perceived the inverted139 image of a ship beneath the image, as shown in Fig. 32; but Dr. Vince, who afterwards observed this phenomenon under a greater variety of forms, found that the ship, which was here considered the real one, was only an erect image of the real ship, which was at that time beneath the horizon, and wholly invisible.

Fig. 33.

In August, 1798, Dr. Vince observed a great variety of these aërial images of vessels approaching the horizon. Sometimes there was seen only one inverted image above the real ship, and this was generally the case when the real ship was full140 in view. But when the real ship was just begin beginning to show its top-mast above the horizon, as at A, Fig. 33, two aërial images of it were seen, one at B inverted, and the other in its natural position at C. In this case the sea was distinctly visible between the erect and inverted images, but in other cases the hull of the one image was immediately in contact with the hull of the other.

Analogous phenomena were seen by Captain Scoresby when navigating with the ship Baffin the icy sea in the immediate neighbourhood of West Greenland. On the 28th of June, 1820, he observed about eighteen sail of ships at the distance of ten or fifteen miles. The sun had shone during the day without the interposition of a cloud, and its rays were peculiarly powerful. The intensity of its light occasioned a painful sensation in the eyes, while its heat softened the tar in the rigging of the ship, and melted the snow on the surrounding ice with such rapidity that pools of fresh water were formed on almost every place, and thousands of rills carried the excess into the sea. There was scarcely a breath of wind: the sea was as smooth as a mirror. The surrounding ice was crowded together, and exhibited every variety, from the smallest lumps to the most magnificent sheets. Bears traversed the fields and floes in unusual numbers, and many whales sported in the recesses and openings among the drift ice. About six in the evening, a light breeze at N.W. having sprung up, a thin stratus or “fog bank,” at first considerably illuminated by the sun, appeared in the same quarter, and gradually rose to the altitude of about a quarter of a degree. At this time most of the ships navigating at the distance of ten or141 fifteen miles began to change their form and magnitude, and when examined by a telescope from the mast-head, exhibited some extraordinary appearances, which differed at almost every point of the compass. One ship had a perfect image, as dark and distinct as the original, united to its mast-head in a reverse position. Two others presented two distinct inverted images in the air, one of them a perfect figure of the original, and the other wanting the hull. Two or three more were strangely distorted, their masts appearing of at least twice their proper height, the top-gallant mast forming one-half of the total elevation; and other vessels exhibited an appearance totally different from all the preceding, being as it were compressed, in place of elongated. Their masts seemed to be scarcely one-half of their proper altitude, in consequence of which one would have supposed that they were greatly heeled-to one side, or in the position called careening. Along with all the images of the ships a reflexion of the ice, sometimes in two strata, also appeared in the air, and these reflexions suggested the idea of cliffs composed of vertical columns of alabaster.

On the 15th, 16th, and 17th of the same month, Mr. Scoresby observed similar phenomena, sometimes extending continuously through half the circumference of the horizon, and at other times appearing only in detached spots in various quarters. The inverted images of distant vessels were often seen in the air, while the ships themselves were far beyond the reach of vision. Some ships were elevated to twice their proper height, while others were compressed almost to a line. Hummocks of ice were surprisingly enlarged, and142 every prominent object in a proper position was either magnified or distorted.

But of all the phenomena witnessed by Mr. Scoresby, that of the Enchanted Coast, as it may be called, must have been the most remarkable. This singular effect was seen on the 18th of July, when the sky was clear, and a tremulous and perfectly transparent vapour was particularly sensible and profuse: at nine o’clock in the morning, when the phenomenon was first seen, the thermometer stood at 42° Fahr.; but in the preceding evening it must have been greatly lower, as the sea was in many places covered with a considerable pellicle of new ice,—a circumstance, which, in the very warmest time of the year, must be considered as quite extraordinary, especially when it is known that 10° farther to the north no freezing of the sea at this season had ever before been observed. Having approached on this occasion so near the unexplored shore of Greenland that the land appeared distinct and bold, Mr. Scoresby was anxious to obtain a drawing of it; but on making the attempt he found that the outline was constantly changing, and he was induced to examine the coast with a telescope, and to sketch the various appearances which presented themselves. These are shown, without any regard to their proper order, in Fig. 34, which we shall describe in Mr. Scoresby’s own words: “The general telescopic appearance of the coast was that of an extensive ancient city abounding with the ruins of castles, obelisks, churches, and monuments, with other large and conspicuous buildings. Some of the hills seemed to be surmounted by turrets, battlements, spires, and pin143nacles; while others, subjected to one or two reflexions, exhibited large masses of rock, apparently suspended in the air, at a considerable elevation above the actual termination of the mountains to which they referred. The whole exhibition was a grand phantasmagoria. Scarcely was any particular portion sketched before it changed its appearance, and assumed the form of an object totally different. It was perhaps alternately a castle, a cathedral, or an obelisk; then expanding horizontally, and coalescing with the adjoining hills, united the intermediate valleys, though some miles in width, by a bridge of a single arch, of the most magnificent appearance and extent. Notwithstanding these repeated changes, the various figures represented in the drawing had all the distinctness of reality; and not only the different strata, but also the veins of the rocks, with the wreaths of snow occupying ravines and fissures, form sharp and distinct lines,144 and exhibited every appearance of the most perfect solidity.”

Fig. 34.

One of the most remarkable facts respecting aërial images presented itself to Mr. Scoresby in a later voyage which he performed to the coast of Greenland in 1822. Having seen an inverted image of a ship in the air, he directed to it his telescope; he was able to discover it to be his father’s ship, which was at the time below the horizon. “It was,” says he, “so well defined, that I could distinguish by a telescope every sail, the general rig of the ship, and its particular character; insomuch, that I confidently pronounced it to be my father’s ship, the Fame, which it afterwards proved to be; though, on comparing notes with my father, I found that our relative position, at the time, gave a distance from one another very nearly thirty miles, being about seventeen miles beyond the horizon, and some leagues beyond the limit of direct vision. I was so struck with the peculiarity of the circumstance, that I mentioned it to the officer of the watch, stating my full conviction that the Fame was then cruising in the neighbouring inlet.”

Several curious effects of the mirage were observed by Baron Humboldt during his travels in South America. When he was residing at Cumana, he frequently saw the islands of Picuita and Boracha suspended in the air, and sometimes with an inverted image. On one occasion he observed small fishing-boats swimming in the air, during more than three or four minutes, above the well-defined horizon of the sea; and when they were viewed through a telescope, one of the boats had an inverted image accompanying it in145 its movements. This distinguished traveller observed similar phenomena in the barren steppes of the Caraccas, and on the borders of the Orinoco, where the river is surrounded by sandy plains. Little hills and chains of hills appeared suspended in the air, when seen from the steppes, at three or four leagues’ distance. Palm-trees standing single in the Llanos appeared to be cut off at bottom, as if a stratum of air separated them from the ground; and, as in the African desert, plains destitute of vegetation appeared to be rivers or lakes. At the Mesa de Pavona M. Humboldt and M. Bonpland saw cows suspended in the air at the distance of 1000 toises, and having their feet elevated 3’ 20” above the soil. In this case the images were erect, but the travellers learned from good authority that inverted images of horses had been seen suspended in the air near Calabozo.

In all these cases of aërial spectres, the images were directly above the real object; but a curious case was observed by Messrs. Jurine and Soret on the 17th September, 1818, where the image of the vessel was on one side of the real one. About 10 P.M. a barque at the distance of about 4000 toises from Bellerive, on the lake of Geneva, was seen approaching to Geneva by the left bank of the lake, and at the same time an image of the sails was observed above the water, which, instead of following the direction of the barque, separated from it, and appeared to approach Geneva by the right bank of the lake, the image moving from east to west, while the barque moved from north to south. When the image first separated from the barque they had both the same146 magnitude, but the image diminished as it receded from it, and was reduced to one-half when the phenomenon disappeared.

A very unusual example of aërial spectres occurred to Dr. A. P. Buchan while walking on the cliff about a mile to the east of Brighton, on the morning of the 28th of November, 1804. “While watching the rising of the sun,” says he, “I turned my eyes directly towards the sea, just as the solar disk emerged from the surface of the water, and saw the face of the cliff on which I was standing represented precisely opposite to me at some distance on the ocean. Calling the attention of my companion to this appearance, we discerned our own figures standing on the summit of the apparent opposite cliff, as well as the representation of the windmill near at hand.

“The reflected images were most distinct precisely opposite to where we stood, and the false cliff seemed to fade away, and to draw near to the real one, in proportion as it receded towards the west. This phenomenon lasted about ten minutes, or till the sun had risen nearly his own diameter above the surface of the ocean. The whole then seemed to be elevated into the air, and successively disappeared, giving an impression very similar to that which is produced by the drawing up of a drop-scene in a theatre. The horizon was cloudy, or perhaps it might with more propriety be said that the surface of the sea was covered with a dense fog of many yards in height, and which gradually receded before the rays of the sun.”

An illusion of a different kind, though not less interesting, is described by the Rev. Mr.147 Hughes in his Travels in Greece, as seen from the summit of Mount Ætna. “I must not forget to mention,” says he, “one extraordinary phenomenon, which we observed, and for which I have searched in vain for a satisfactory solution. At the extremity of the vast shadow which Ætna projects across the island, appeared a perfect and distinct image of the mountain itself elevated above the horizon, and diminished as if viewed in a concave mirror. Where or what the reflector could be which exhibited this image, I cannot conceive; we could not be mistaken in its appearance, for all our party observed it, and we had been prepared for it beforehand by our Catanian friends. It remained visible about ten minutes, and disappeared as the shadow decreased. Mr. Jones observed the same phenomenon, as well as some other friends with whom I conversed upon the subject in England.”

It is impossible to study the preceding phenomena without being impressed with the conviction, that nature is full of the marvellous, and that the progress of science and the diffusion of knowledge are alone capable of dispelling the fears which her wonders must necessarily excite even in enlightened minds. When a spectre haunts the couch of the sick, or follows the susceptible vision of the invalid, a consciousness of indisposition divests the apparition of much of its terror, while its invisibility to surrounding friends soon stamps it with the impress of a false perception. The spectres of the conjuror, too, however skilfully they may be raised, quickly lose their supernatural character; and even the most ignorant beholder regards the modern148 magician as but an ordinary man, who borrows from the sciences the best working implements of his art. But when, in the midst of solitude, and in situations where the mind is undisturbed by sublunary cares, we see our own image delineated in the air, and mimicking in gigantic perspective the tiny movements of humanity;—when we see troops in military array performing their evolutions on the very face of an almost inaccessible precipice—when, in the eye of day, a mountain seems to become transparent, and exhibits on one side of it a castle which we know to exist only on the other; when distant objects, concealed by the roundness of the earth, and beyond the cognisance of the telescope, are actually transferred over the intervening convexity and presented in distinct and magnified outline to our accurate examination;—when such varied and striking phantasms are seen also by all around us, and therefore appear in the character of real phenomena of nature, our impressions of supernatural agency can only be removed by a distinct and satisfactory knowledge of the causes which gave them birth.

It is only within the last forty years that science has brought these atmospherical spectres within the circle of her dominion; and not only are all their phenomena susceptible of distinct explanation, but we can even reproduce them on a small scale with the simplest elements of our optical apparatus.

Fig. 35.

In order to convey a general idea of the causes of these phenomena, let ABCD, Fig. 35, be a glass trough filled with water, and let a small ship be placed at S. An eye situated about E,149 will see the top-mast of the ship S, directly through the plate of glass BD. Fix a convex lens a of short focus upon the plate of glass BD, and a little above a straight line SE joining the ship and the eye; and immediately above the convex lens a place a concave one b. The eye will now see, through the convex lens a, an inverted image of the ship at S´, and through the concave lens b, an erect image of the ship at S´´, representing in a general way the phenomena shown in Fig. 33. But it will be asked, where are the lenses in nature to produce these effects? This question is easily answered. If we take a tin tube with glass plates at each end, and fill it with water, and if we cool it on the outside with ice, it will act like a concave lens when the cooling effect has reached the axis; and, on the other hand, if we heat the same tube filled with water, on the outside, it will act as a convex glass. In the first case the density of the water diminishes towards the centre, and in the second it increases150 towards the centre. The very same effects are produced in the air, only a greater tract of air is necessary for showing the effect produced, by heating and cooling it unequally. If we now remove the lenses a, b, and hold a heated iron horizontally above the water in the trough ABC, the heat will gradually descend, expanding or rendering rarer the upper portions of the fluid. If, when the heat has reached within a little of the bottom, we look through the trough at the ship S in the direction ES´, we shall see an inverted image at S´, and an erect one at S´´; and if we hide from the eye at E all the ship S, excepting the top-mast, we shall have an exact representation of the phenomenon in Fig. 33. The experiment will succeed better with oil in place of water; and the same result may be obtained without heat, by pouring clear syrup into the glass trough till it is nearly one-third full, and then filling it up with water. The water will gradually incorporate with the syrup, and produce, as Dr. Wollaston has shown, a regular gradation of density, diminishing from that of the pure syrup to that of the pure water. Similar effects may be obtained by using masses of transparent solids, such as glass, rock-salt, &c.

Now it is easy to conceive how the changes of density which we can thus produce artificially may be produced in nature. If, in serene weather, the surface of the sea is much colder than the air of the atmosphere, as it frequently is, and as it was to a very great degree during the phenomena described by Mr. Scoresby, the air next the sea will gradually become colder and colder, by giving out its heat to the water; and the air immediately151 above will give out its heat to the cooler air immediately below it, so that the air from the surface of the sea, to a considerable height upwards, will gradually diminish in density, and therefore must produce the very phenomena we have described.

Fig. 36.

The phenomenon of Dover Castle seen on the Ramsgate side of the hill, was produced by the air being more dense near the ground and above the sea than at greater heights, and hence the rays proceeding from the castle reached the eye in curve lines, and the cause of its occupying its natural position on the hill, and not being seen in the air, was that the top of the hill itself, in consequence of being so near the castle, suffered the same change from the varying density of the air, and therefore the castle and the hill were equally elevated and retained their relative positions. The reason why the image of the castle and hill appeared erect was, that the rays from the top and bottom of the castle had not crossed before they reached Ramsgate; but as they met152 at Ramsgate, an eye at a greater distance from the castle, and in the path of the rays, would have seen the image inverted. This will be better understood from the preceding diagram, which represents the actual progress of the rays, from a ship SP, concealed from the observer at E by the convexity of the earth PQE. A ray proceeding from the keel of the ship P is refracted into the curve line P c x c E, and a ray proceeding from the top-mast S, is refracted in the direction S d x d E, the two rays crossing at x, and proceeding to the eye E with the ray from the keel P uppermost; hence the ship must appear inverted as at s p. Now if the eye E of the observer had been placed nearer the ship as at x, before the rays crossed, as was the case at Ramsgate, it would have seen an erect image of the ship raised a little above the real ship SP. Rays S m, S n, proceeding higher up in the air, are refracted in the directions S m m E, S n n E, but do not cross before they reach the eye, and therefore they afford the erect image of the ship shown at s´ p´.

The aërial troopers seen at Souterfell were produced by the very same process as the spectre of Dover Castle, having been brought by unequal refraction from one side of the hill to the other. It is not our business to discover how a troop of soldiers came to be performing their evolutions on the other side of Souterfell; but if there was then no road along which they could be marching, it is highly probable that they were troops exercising among the hills in secret, previously to the breaking out of the rebellion in 1745.

The image of the Genevese barque which was153 seen sailing at a distance from the real one, arose from the same cause as the images of ships in the air; with this difference only, that in this case the strata of equal density were vertical or perpendicular to the water, whereas, in the former cases they were horizontal or parallel to the water. The state of the air which produced the lateral image may be produced by a headland or island, or even rocks, near the surface, and covered with water. These headlands, islands, or sunken rocks being powerfully heated by the sun in the daytime, will heat the air immediately above them, while the adjacent air over the sea will retain its former coolness and density. Hence there will necessarily arise a gradation of density varying in the same horizontal direction, or where the lines of equal density are vertical. If we suppose the very same state of the air to exist in a horizontal plane which exists in a vertical plane, in Fig. 36, then the same images would be seen in a horizontal line, viz., an inverted one at s p, and an erect one at s´p´. In the case of the Genevese barque, the rays had not crossed before they reached the eye, and therefore the image was an erect one. Had the real Genevese barque been concealed by some promontory or other cause from the observation of Messrs. Jurine and Soret, they might have attached a supernatural character to the spectral image, especially if they had seen it gradually decay, and finally disappear on the still and unbroken surface of the lake. No similar fact had been previously observed, and there were no circumstances in the case to have excited the suspicion that it was the spectre of a real vessel produced by unequal refraction.

154

The spectre of the Brocken and other phenomena of the same kind, have essentially a different origin from those which arise from unequal refraction. They are merely shadows of the observer projected on dense vapour or thin fleecy clouds, which have the power of reflecting much light. They are seen most frequently at sunrise, because it is at that time that the vapours and clouds necessary for their production are most likely to be generated; and they can be seen only when the sun is throwing his rays horizontally, because the shadow of the observer would otherwise be thrown either up in the air, or down upon the ground. If there are two persons looking at the phenomenon, as when M. Haue and the landlord saw it together, each observer will see his own image most distinctly, and the head will be more distinct than the rest of the figure, because the rays of the sun will be more copiously reflected at a perpendicular incidence: and as, from this cause, the light reflected from the vapour or cloud becomes fainter farther from the shadow, the appearance of a halo round the head of the observer is frequently visible. M. Haue mentions the extraordinary circumstance of the two spectres of him and the landlord being joined by a third figure, but he unfortunately does not inform us which of the two figures was doubled, for it is impossible that a person could have joined their party unobserved. It is very probable that the new spectre forms a natural addition to the group, as we have represented it in Fig. 30; and, if this was the case, it could only have been produced by a duplication of one of the figures produced by unequal refraction.

155

The reflected spectre of Dr. Buchan standing upon the cliff at Brighton, arose from a cause to which we have not yet adverted. It was obviously no shadow, for it is certain, from the locality, that the rays of the sun fell upon the face of the cliff and upon his person at an angle of about 73° from the perpendicular, so as to illuminate them strongly. Now, there are two ways in which such an image may have been reflected, namely, either from strata of air of variable density, or from a vertical stratum of vapour, consisting of exceedingly minute globules of water. Whenever light suffers refraction, either in passing at once from one medium into another, or from one part of the same medium into another of different density, a portion of it suffers reflexion. If an object, therefore, were strongly illuminated, a sufficiently distinct image, or rather shadow of it, might be seen by reflexion from strata of air of different density. As the temperature at which moisture is deposited in the atmosphere varies with the density of the air, then at the same temperature moisture might be depositing in a stratum of one density, while no deposition is taking place in the adjacent stratum of a different density. Hence there would exist, as it were in the air, a vertical wall or stratum of minute globules of water, from the surface of which a sufficiently distinct image of a highly illuminated object might be reflected. That this is possible may be proved by breathing upon glass. If the particles deposited upon the glass are large, then no distinct reflection will take place; but if the particles be very small, we shall see a distinct image formed by the surface of the aqueous film.

156

The phenomena of the Fata Morgana have been too imperfectly described to enable us to offer a satisfactory explanation of them. The aërial images are obviously those formed by unequal refraction. The pictures seen on the sea may be either the aërial images reflected from its surface, or from a stratum of dense vapour, or they may be the direct reflexions from the objects themselves. The coloured images, as described by Minasi, have never been seen in any analogous phenomena, and require to be better described before they can be submitted to scientific examination.

The representation of ships in the air by unequal refraction has no doubt given rise in early times to those superstitions which have prevailed in different countries respecting “phantom ships,” as Mr. Washington Irving calls them, which always sail in the eye of the wind, and plough their way through the smooth sea, where there is not a breath of wind upon its surface. In his beautiful story of the storm ship, which makes its way up the Hudson against wind and tide, this elegant writer has finely embodied one of the most interesting superstitions of the early American colonists. The Flying Dutchman had, in all probability, a similar origin; and the wizard beacon-keeper of the Isle of France, who saw in the air the vessels bound to the island long before they appeared in the offing, must have derived his power from a diligent observation of the phenomena of nature.

157

LETTER VII.

Illusions depending on the ear—Practised by the ancients—Speaking and singing heads of the ancients—Exhibition of the Invisible Girl described and explained—Illusions arising from the difficulty of determining the direction of sounds—Singular example of this illusion—Nature of ventriloquism—Exhibitions of some of the most celebrated ventriloquists—M. St. Gille—Louis Brabant—M. Alexandre—Capt. Lyon’s account of Esquimaux ventriloquists.

Next to the eye, the ear is the most fertile source of our illusions, and the ancient magicians seem to have been very successful in turning to their purposes the doctrines of sound. In the Labyrinth of Egypt, which contained twelve palaces and 1500 subterraneous apartments, the gods were made to speak in a voice of thunder; and Pliny, in whose time this singular structure existed, informs us, that some of the palaces were so constructed that their doors could not be opened without permitting the peals of thunder to be heard in the interior. When Darius Hystaspes ascended the throne, and allowed his subjects to prostrate themselves before him as a god, the divinity of his character was impressed upon his worshippers by the bursts of thunder and flashes of lightning which accompanied their devotion. History has of course not informed us how these effects were produced; but it is probable that, in the subterraneous and vaulted apartments of the Egyptian labyrinth, the reverberated sounds arising from the mere opening158 and shutting of the doors themselves afforded a sufficient imitation of ordinary thunder. In the palace of the Persian king, however, a more artificial imitation is likely to have been employed, and it is not improbable that the method used in our modern theatres was known to the ancients. A thin sheet of iron, three or four feet long, such as that used for German stoves, is held by one corner between the finger and the thumb, and allowed to hang freely by its own weight. The hand is then moved or shaken horizontally, so as to agitate the corner in a direction at right angles to the surface of the sheet. By this simple process a great variety of sounds may be produced, varying from the deep growl of distant thunder to those loud and explosive bursts which rattle in quick succession from clouds immediately over our heads. The operator soon acquires great power over this instrument, so as to be able to produce from it any intensity and character of sound that may be required. The same effect may be produced by sheets of tin-plate, and by thin plates of mica; but, on account of their small size, the sound is shorter and more acute. In modern exhibitions an admirable imitation of lightning is produced by throwing the powder of rosin, or the dust of lycopodium, through a flame; and the rattling showers of rain which accompany these meteors are well imitated by a well-regulated shower of peas.

The principal pieces of acoustic mechanism used by the ancients were speaking or singing heads, which were constructed for the purpose of representing the gods, or of uttering oracular responses. Among these, the speaking head of159 Orpheus, which uttered its responses at Lesbos, is one of the most famous. It was celebrated not only throughout Greece, but even in Persia; and it had the credit of predicting, in the equivocal language of the heathen oracles, the bloody death which terminated the expedition of Cyrus the Great into Scythia. Odin, the mighty magician of the North, who imported into Scandinavia the magical arts of the East, possessed a speaking head, said to be that of the sage Minos, which he had enchased in gold, and which uttered responses that had all the authority of a divine revelation. The celebrated mechanic Gerbert, who filled the papal chair A.D. 1000, under the name of Sylvester II., constructed a speaking head of brass. Albertus Magnus is said to have executed a head in the thirteenth century, which not only moved but spoke. It was made of earthenware, and Thomas Aquinas is said to have been so terrified when he saw it, that he broke it in pieces; upon which the mechanist exclaimed, “There goes the labour of thirty years!”

It has been supposed by some authors, that in the ancient speaking-machines the deception is effected by means of ventriloquism, the voice issuing from the juggler himself; but it is more probable that the sound was conveyed by pipes from a person in another apartment to the mouth of the figure. Lucian, indeed, expressly informs us, that the impostor Alexander made his figure of Æsculapius speak, by transmitting his voice through the gullet of a crane to the mouth of the statue; and that this method was general appears from a passage in Theodoretus, who assures us, that in the fourth century, when Bishop Theo160philus broke to pieces the statues at Alexandria, he found some which were hollow, and which were so placed against a wall, that the priest could conceal himself behind them; and address the ignorant spectators through their mouths.

Even in modern times, speaking-machines have been constructed on this principle. The figure is frequently a mere head placed upon a hollow pedestal, which, in order to promote the deception, contains a pair of bellows, a sounding-board, a cylinder and pipes supposed to represent the organs of speech. In other cases these are dispensed with, and a simple wooden head utters its sounds through a speaking trumpet. At the court of Charles II., this deception was exhibited with great effect by one Thomas Irson, an Englishman; and when the astonishment had become very general, a popish priest was discovered by one of the pages in an adjoining apartment. The questions had been proposed to the wooden figure by whispering into its ear, and this learned personage had answered them all with great ability, by speaking through a pipe in the same language in which the questions were proposed. Professor Beckmann informs us that children and women were generally concealed either in the juggler’s box or in the adjacent apartment, and that the juggler gave them every assistance by means of signs previously agreed upon. When one of these exhibitions was shown at Göttingen, the Professor was allowed, on the promise of secrecy, to witness the process of deception. He saw the assistant in another room, standing before the pipe with a card in his hand, upon which the signs agreed upon had161 been marked, and he had been introduced so privately into the house that even the landlady was ignorant of his being there.

An exhibition of the very same kind has been brought forward in our own day, under the name of the Invisible Girl; and as the mechanism employed was extremely ingenious, and is well fitted to convey an idea of this class of deceptions, we shall give a detailed description of it.

Fig. 37.

The machinery, as constructed by M. Charles, is shown in fig. 37 in perspective, and a plan of it in Fig. 38. The four upright posts A, A, A, A, are united at top by a cross rail B, B, and by two similar rails at bottom. Four bent wires a, a, a, a, proceeded from the top of these posts, and terminated at c. A hollow copper ball M, about a foot in diameter, was suspended from these wires by four slender ribands b, b, b, b, and into the copper ball were fixed the extremities of162 four trumpets T, T, T, T, with their mouths outwards.

Fig. 38.

The apparatus now described was all that was visible to the spectator; and though fixed in one spot, yet it had the appearance of a piece of separate machinery, which might have occupied any other part of the room. When one of the spectators was requested by the exhibitor to propose some question, he did it by speaking into one of the trumpets at T. An appropriate answer was then returned from all the trumpets, and the sound issued with sufficient intensity to be heard by an ear applied to any of them, and yet it was so weak that it appeared to come from a person of very diminutive size. Hence the sound was supposed to come from an invisible girl, though the real speaker was a full-grown woman. The invisible lady conversed in different languages, sang beautifully, and made the most lively and appropriate remarks on the persons in the room.

This exhibition was obviously far more wonderful than the speaking heads which we have163 described, as the latter invariably communicated with a wall, or with a pedestal through which pipes could be carried into the next apartment. But the ball M and its trumpets communicated with nothing through which sound could be conveyed. The spectator satisfied himself by examination that the ribands b, b, were real ribands, which concealed nothing, and which could convey no sound; and as he never conceived that the ordinary piece of frame-work AB could be of any other use than its apparent one of supporting the sphere M, and defending it from the spectators, he was left in utter amazement respecting the origin of the sound, and his surprise was increased by the difference between the sounds which were uttered and those of ordinary speech.

Though the spectators were thus deceived by their own reasoning, yet the process of deception was a very simple one. In two of the horizontal railings A, A, Fig. 38, opposite the trumpet mouths T, there was an aperture communicating with a pipe or tube which went to the vertical post B, and descending it, as shown at TAA, Fig. 39, went beneath the floor f f, in the direction p p, and entered the apartment N, where the invisible lady sat. On the side of the partition about h, there was a small hole through which the lady saw what was going on in the exhibition-room, and communications were no doubt made to her by signals from the person who attended the machine. When one of the spectators asked a question by speaking into one of the trumpets T, the sound was reflected from the mouth of the trumpet back to the aperture at A, in the horizontal rail, Fig. 38, and was dis164tinctly conveyed along the closed tube into the apartment N. In like manner the answer issued from the aperture A, and being reflected back to the ear of the spectator by the trumpet, he heard the sounds with that change of character which they receive when transmitted through a tube and then reflected to the ear.

Fig. 39.

The surprise of the auditors was greatly increased by the circumstance, that an answer was returned to questions put in a whisper, and also by the conviction that nobody but a person in the middle of the audience could observe the circumstances to which the invisible figure frequently adverted.

Although the performances of speaking heads were generally effected by the methods now described, yet there is reason to think that the ventriloquist sometimes presided at the exhibition, and deceived the audience by his extraordinary powers of illusion. There is no species of deception more irresistible in its effects than that which arises from the uncertainty with which we judge165 of the direction and distance of sounds. Every person must have noticed how a sound in their own ears is often mistaken for some loud noise moderated by the distance from which it is supposed to come; and the sportsman must have frequently been surprised at the existence of musical sounds humming remotely in the extended heath, when it was only the wind sounding in the barrel of his gun. The great proportion of apparitions that haunt old castles and apartments associated with death, exist only in the sounds which accompany them. The imagination even of the boldest inmate of a place hallowed by superstition, will transfer some trifling sound near his own person to a direction and to a distance very different from the truth, and the sound which otherwise might have no peculiar complexion will derive another character from its new locality. Spurning the idea of a supernatural origin, he determines to unmask the spectre, and grapple with it in its den. All the inmates of the house are found to be asleep—even the quadrupeds are in their lair—there is not a breath of wind to ruffle the lake that reflects through the casement the reclining crescent of the night; and the massive walls in which he is enclosed forbid the idea that he has been disturbed by the warping of panelling or the bending of partitions. His search is vain; and he remains master of his own secret, till he has another opportunity of investigation. The same sound again disturbs him, and, modified probably by his own position at the time, it may perhaps appear to come in a direction slightly different from the last. His searches are resumed,166 and he is again disappointed. If this incident should recur night after night with the same result—if the sound should appear to depend upon his own motions, or be any how associated with himself, with his present feelings, or with his past history, his personal courage will give way; a superstitious dread, at which he himself perhaps laughs, will seize his mind; and he will rather believe that the sounds have a supernatural origin, than that they could continue to issue from a spot where he knows there is no natural cause for their production.

I have had occasion to have personal knowledge of a case much stronger than that which has now been put. A gentleman, devoid of all superstitious feelings, and living in a house free from any gloomy associations, heard night after night in his bed-room a singular noise, unlike any ordinary sound to which he was accustomed. He had slept in the same room for years without hearing it, and he attributed it at first to some change of circumstances in the roof or in the walls of the room, but after the strictest examination no cause could be found for it. It occurred only once in the night; it was heard almost every night, with few interruptions. It was over in an instant, and it never took place till after the gentleman had gone to bed. It was always distinctly heard by his companion, to whose time of going to bed it had no relation. It depended on the gentleman alone, and it followed him into another apartment with another bed, on the opposite side of the house. Accustomed to such investigations, he made the most diligent but fruitless search into its cause. The consideration167 that the sound had a special reference to him alone, operated upon his imagination, and he did not scruple to acknowledge that the recurrence of the mysterious sound produced a superstitious feeling at the moment. Many months afterwards it was found that the sound arose from the partial opening of the door of a wardrobe which was within a few feet of the gentleman’s head, and which had been taken into the other apartment. This wardrobe was almost always opened before he retired to bed, and the door being a little too tight, it gradually forced itself open with a sort of dull sound, resembling the note of a drum. As the door had only started half an inch out of its place, its change of position never attracted attention. The sound, indeed, seemed to come in a different direction, and from a greater distance.

When sounds so mysterious in their origin are heard by persons predisposed to a belief in the marvellous, their influence over the mind must be very powerful. An inquiry into their origin, if it is made at all, will be made more in the hope of confirming than of removing the original impression, and the unfortunate victim of his own fears will also be the willing dupe of his own judgment.

This uncertainty with respect to the direction of sound is the foundation of the art of ventriloquism. If we place ten men in a row at such a distance from us that they are included in the angle within which we cannot judge of the direction of sound, and if in a calm day each of them speaks in succession, we shall not be able with closed eyes to determine from which of the168 ten men any of the sounds proceed, and we shall be incapable of perceiving that there is any difference in the direction of the sounds emitted by the two outermost. If a man and a child are placed within the same angle, and if the man speaks with the accent of a child without any corresponding motion in his mouth or face, we shall necessarily believe that the voice comes from the child; nay, if the child is so distant from the man that the voice actually appears to us to come from the man, we shall still continue in the belief that the child is the speaker; and this conviction would acquire additional strength if the child favoured the deception, by accommodating its features and gestures to the words spoken by the man. So powerful, indeed, is the influence of this deception, that if a jack-ass, placed near the man, were to open its mouth, and shake its head responsive to the words uttered by his neighbour, we should rather believe that the ass spoke than that the sounds proceeded from a person whose mouth was shut, and the muscles of whose face were in perfect repose. If our imagination were even directed to a marble statue or a lump of inanimate matter, as the source from which we were to expect the sounds to issue, we would still be deceived, and would refer the sounds even to these lifeless objects. The illusion would be greatly promoted, if the voice were totally different in its tone and character from that of the man from whom it really comes; and if he occasionally speaks in his own full and measured voice, the belief will be irresistible that the assumed voice proceeds from the quadruped or from the inanimate object.

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When the sounds which are required to proceed from any given object are such as they are actually calculated to yield, the process of deception is extremely easy; and it may be successfully executed, even if the angle between the real and the supposed direction of the sound is much greater than the angle of uncertainty. Mr. Dugald Stewart has stated some cases in which deceptions of this kind were very perfect. He mentions his having seen a person who, by counterfeiting the gesticulations of a performer on the violin, while he imitated the music by his voice, riveted the eyes of his audience on the instrument, though every sound they heard proceeded from his own mouth. The late Savile Carey, who imitated the whistling of the wind through a narrow chink, told Mr. Stewart that he had frequently practised this deception in the corner of a coffee-house, and that he seldom failed to see some of the company rise to examine the tightness of the windows, while others, more intent on their newspapers, contented themselves with putting on their hats and buttoning their coats. Mr. Stewart likewise mentions an exhibition formerly common in some of the continental theatres, where a performer on the stage displayed the dumb-show of singing with his lips and eyes and gestures, while another person unseen supplied the music with his voice. The deception in this case he found to be at first so complete as to impose upon the nicest ear and the quickest eye; but in the progress of the entertainment, he became distinctly sensible of the imposition, and sometimes wondered that it should have misled him for a moment. In this170 case there can be no doubt that the deception was at first the work of the imagination, and was not sustained by the acoustic principle. The real and the mock singer were too distant, and when the influence of the imagination subsided, the true direction of the sound was discovered. This detection of the imposture, however, may have arisen from another cause. If the mock singer happened to change the position of his head, while the real singer made no corresponding change in his voice, the attentive spectator would at once notice this incongruity, and discover the imposition.

In many of the feats of ventriloquism the performer contrives, under some pretence or other, to conceal his face, but ventriloquists of great distinction, such as M. Alexandre, practise their art without any such concealment.

Ventriloquism loses its distinctive character if its imitations are not performed by a voice from the belly. The voice, indeed, does not actually come from that region; but when the ventriloquist utters sounds from the larynx without moving the muscles of his face, he gives them strength by a powerful action of the abdominal muscles. Hence he speaks by means of his belly, although the throat is the real source from whence the sounds proceed. Mr. Dugald Stewart has doubted the fact, that ventriloquists possess the power of fetching a voice from within: he cannot conceive what aid could be derived from such an extraordinary power; and he considers that the imagination, when seconded by such powers of imitation as some mimics possess, is quite sufficient to account for all the phenomena171 of ventriloquism which he has heard. This opinion, however, is strongly opposed by the remark made to Mr. Stewart himself by a ventriloquist, “that his art would be perfect, if it were possible only to speak distinctly without any movement of the lips at all.” But, independent of this admission, it is a matter of absolute certainty, that this internal power is exercised by the true ventriloquist. In the account which the Abbé Chapelle has given of the performances of M. St. Gille and Louis Brabant, he distinctly states that M. St. Gille appeared to be absolutely mute while he was exercising his art, and that no change in his countenance could be discovered.16 He affirms, also, that the countenance of Louis Brabant exhibited no change, and that his lips were close and inactive. M. Richerand, who attentively watched the performances of M. Fitz-James, assures us that during his exhibition there was a distention in the epigastric region, and that he could not long continue the exertion without fatigue.

The influence over the human mind which the ventriloquist derives from the skilful practice of his art is greater than that which is exercised by any other species of conjuror. The ordinary magician requires his theatre, his accomplices, and the instruments of his art, and he enjoys but a local sovereignty within the precincts of his own magic circle. The ventriloquist, on the contrary, has the supernatural always at his command. In the open fields as well as in the crowded city, in the private apartment as well as in the public hall, he can summon up innumerable172 spirits; and though the persons of his fictitious dialogue are not visible to the eye, yet they are unequivocally present to the imagination of his auditors, as if they had been shadowed forth in the silence of a spectral form. In order to convey some idea of the influence of this illusion, I shall mention a few well-authenticated cases of successful ventriloquism.

M. St. Gille, a grocer of St. Germain-en-Laye, whose performances have been recorded by the Abbé de la Chapelle, had occasion to shelter himself from a storm in a neighbouring convent, where the monks were in deep mourning for a much-esteemed member of their community who had been recently buried. While lamenting over the tomb of their deceased brother the slight honours which had been paid to his memory, a voice was suddenly heard to issue from the roof of the choir bewailing the condition of the deceased in purgatory, and reproving the brotherhood for their want of zeal. The tidings of this supernatural event brought the whole brotherhood to the church. The voice from above repeated its lamentations and reproaches, and the whole convent fell upon their faces, and vowed to make a reparation of their error. They accordingly chanted in full choir a De Profundis, during the intervals of which the spirit of the departed monk expressed his satisfaction at their pious exercises. The prior afterwards inveighed against modern scepticism on the subject of apparitions, and M. St. Gille had great difficulty in convincing the fraternity that the whole was a deception.

On another occasion, a commission of the Royal173 Academy of Sciences at Paris, attended by several persons of the highest rank, met at St. Germain-en-Laye to witness the performances of M. St. Gille. The real object of their meeting was purposely withheld from a lady of the party, who was informed that an aërial spirit had lately established itself in the neighbourhood, and that the object of the assembly was to investigate the matter. When the party had sat down to dinner in the open air, the spirit addressed the lady in a voice which seemed to come from above their heads, from the surface of the ground at a great distance, or from a considerable depth under her feet. Having been thus addressed at intervals during two hours, the lady was firmly convinced of the existence of the spirit, and could with difficulty be undeceived.

Another ventriloquist, Louis Brabant, who had been valet-de-chambre to Francis I., turned his powers to a more profitable account. Having fallen in love with a rich and beautiful heiress, he was rejected by her parents as an unsuitable match for their daughter. On the death of her father, Louis paid a visit to the widow, and he had no sooner entered the house than she heard the voice of her deceased husband addressing her from above, “Give my daughter in marriage to Louis Brabant, who is a man of large fortune and excellent character. I endure the inexpressible torments of purgatory for having refused her to him. Obey this admonition, and give everlasting repose to the soul of your poor husband.” This awful command could not be resisted, and the widow announced her compliance with it.

As our conjuror, however, required money for the completion of his marriage, he resolved to174 work upon the fears of one Cornu, an old banker at Lyons, who had amassed immense wealth by usury and extortion. Having obtained an interview with the miser, he introduced the subjects of demons and spectres, and the torments of purgatory; and, during an interval of silence, the voice of the miser’s deceased father was heard complaining of his dreadful situation in purgatory, and calling upon his son to rescue him from his sufferings by enabling Louis Brabant to redeem the Christians that were enslaved by the Turks. The awe-struck miser was also threatened with eternal damnation if he did not thus expiate his own sins; but such was the grasp that the banker took of his gold, that the ventriloquist was obliged to pay him another visit. On this occasion, not only his father but all his deceased relatives appealed to him in behalf of his own soul and theirs; and such was the loudness of their complaints, that the spirit of the banker was subdued, and he gave the ventriloquist ten thousand crowns to liberate the Christian captives. When the miser was afterwards undeceived, he is said to have been so mortified that he died of vexation.

The ventriloquists of the nineteenth century made great additions to their art, and the performances of M. Fitz-James and M. Alexandre, which must have been seen by many of our countrymen, were far superior to those of their predecessors. Besides the art of speaking by the muscles of the throat and the abdomen, without moving those of the face, these artists had not only studied with great diligence and success the modifications which sounds of all kinds undergo from distance, obstructions, and other causes, but had acquired the art of imitating them in the175 highest perfection. The ventriloquist was therefore able to carry on a dialogue in which the dramatis voces, as they may be called, were numerous; and when on the outside of an apartment, he could personate a mob with its infinite variety of noise and vociferation. Their influence over an audience was still further extended by a singular power over the muscles of the body. M. Fitz-James actually succeeded in making the opposite or corresponding muscles act differently from each other; and while one side of his face was merry and laughing, the other was full of sorrow and in tears. At one moment he was tall, thin, and melancholic, and after pausing behind a screen, he came out “bloated with obesity and staggering with fulness.” M. Alexandre possessed the same power over his face and figure; and so striking was the contrast of two of these forms, that an excellent sculptor, Mr. Joseph, has perpetuated them in marble.

This new acquirement of the ventriloquist enabled him, in his own single person and with his own single voice, to represent upon the stage a dramatic composition which would have required the assistance of several actors. Although only one character in the piece could be seen at the same time, yet they all appeared during its performance, and the change of face and figure on the part of the ventriloquist was so perfect, that his personal identity could not be recognized in the dramatis personæ. This deception was rendered still more complete by a particular construction of the dresses, which enabled the performer to reappear in a new character after an interval so short that the audience necessarily believed that it was another person.

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It is a curious circumstance that Captain Lyon found among the Esquimaux of Igloolik ventriloquists of no mean skill. There is much rivalry among the professors of the art, who do not expose each other’s secrets, and their exhibitions derive great importance from the rarity of their occurrence. The following account of one of them is so interesting that we shall give the whole of it in Captain Lyon’s words:—

“Amongst our Igloolik acquaintances were two females and a few male wizards, of whom the principal was Toolemak. This personage was cunning and intelligent; and, whether professionally, or from his skill in the chase, but perhaps from both reasons, was considered by all the tribe as a man of importance. As I invariably paid great deference to his opinion on all subjects connected with his calling, he freely communicated to me his superior knowledge, and did not scruple to allow of my being present at his interviews with Tornga, or his patron spirit. In consequence of this, I took an early opportunity of requesting my friend to exhibit his skill in my cabin. His old wife was with him, and by much flattery and an accidental display of a glittering knife and some beads, she assisted me in obtaining my request. All light excluded, our sorcerer began chanting to his wife with great vehemence, and she in return answered by singing the Amna-aya, which was not discontinued during the whole ceremony. As far as I could hear, he afterwards began turning himself rapidly round, and in a loud, powerful voice vociferated for Tornga with great impatience, at the same time blowing and snorting like a walrus. His noise, impatience, and agitation increased every177 moment, and he at length seated himself on the deck, varying his tones, and making a rustling with his clothes. Suddenly the voice seemed smothered, and was so managed as to sound as if retreating beneath the deck, each moment becoming more distant, and ultimately giving the idea of being many feet below the cabin, when it ceased entirely. His wife now, in answer to my queries, informed me very seriously, that he had dived, and that he would send up Tornga. Accordingly, in about half a minute, a distant blowing was heard very slowly approaching, and a voice, which differed from that at first heard, was at times mingled with the blowing, until at length both sounds became distinct, and the old woman informed me that Tornga was come to answer my questions. I accordingly asked several questions of the sagacious spirit, to each of which inquiries I received an answer by two loud claps on the deck, which I was given to understand were favourable.

“A very hollow, yet powerful voice, certainly much different from the tones of Toolemak, now chanted for some time, and a strange jumble of hisses, groans, shouts, and gabblings like a turkey, succeeded in rapid order. The old woman sang with increased energy; and as I took it for granted that this was all intended to astonish the Kabloona, I cried repeatedly that I was very much afraid. This, as I expected, added fuel to the fire, until the poor immortal, exhausted by its own might, asked leave to retire.

“The voice gradually sank from our hearing as at first, and a very indistinct hissing succeeded; in its advance it sounded like the tone produced178 by the wind on the brass chord of an Æolian harp. This was soon changed to a rapid hiss like that of a rocket, and Toolemak with a yell announced his return. I had held my breath at the first distant hissing, and twice exhausted myself, yet our conjuror did not once respire, and even his returning and powerful yell was uttered without a previous stop or inspiration of air.

“Light being admitted, our wizard, as might be expected, was in a profuse perspiration, and certainly much exhausted by his exertions, which had continued for at least half an hour. We now observed a couple of bunches, each consisting of two stripes of white deer-skin, and a long piece of sinew, attached to the back of his coat. These we had not seen before, and were informed that they had been sewn on by Tornga while he was below.”17

Captain Lyon had the good fortune to witness another of Toolemak’s exhibitions, and he was much struck with the wonderful steadiness of the wizard throughout the whole performance, which lasted an hour and a half. He did not once appear to move, for he was so close to the skin behind which Captain Lyon sat, that if he had done so he must have perceived it. Captain Lyon did not hear the least rustling of his clothes, or even distinguish his breathing, although his outcries were made with great exertion.18

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LETTER VIII.

Musical and harmonic sounds explained—Power of breaking glasses with the voice—Musical sounds from the vibration of a column of air—and of solid bodies—Kaleidophone—Singular acoustic figures produced on sand laid on vibrating plates of glass—and on stretched membranes—Vibration of flat rulers and cylinders of glass—Production of silence from two sounds—Production of darkness from two lights—Explanation of these singular effects—Acoustic automaton—Droz’s bleating sheep—Maillardet’s singing-bird—Vaucanson’s flute-player—His pipe and tabor-player—Baron Kempelen’s talking-engine—Kratzenstein’s speaking-machine—Mr. Willis’s researches.

Among the discoveries of modern science, there are few more remarkable than those which relate to the production of harmonic sounds. We are all familiar with the effects of musical instruments, from the deep-toned voice of the organ to the wiry shrill of the Jew’s harp. We sit entranced under their magical influence, whether the ear is charmed with the melody of their sounds, or the heart agitated by the sympathies which they rouse. But though we may admire their external form, and the skill of the artist who constructed them, we never think of inquiring into the cause of such extraordinary combinations.

Sounds of all kinds are conveyed to the organ of hearing through the air; and if this element were to be destroyed, all nature would be buried in the deepest silence. Noises of every variety,180 whether they are musical or discordant, high or low, move through the air of our atmosphere at the surface of the earth with a velocity of 1090 feet in a second, or 765 miles per hour; but in sulphurous acid gas sound moves only through 751 feet in a second, while in hydrogen gas it moves with the great velocity of 3000 feet. Along fluid and solid bodies, its progress is still more rapid. Through water it moves at the rate of 4708 feet in a second, through tin at the rate of 8175 feet, and through iron, glass, and some kinds of wood, at the rate of 18,530 feet.

When a number of single and separate sounds follow each other in rapid succession, they produce a continued sound, in the same manner as a continuous circle of light is produced by whirling round a burning stick before the eye. In order that the sound may appear a single one to the ear, nearly sixteen separate sounds must follow one another every second. When these sounds are exactly similar, and recur at equal intervals, they form a musical sound. In order to produce such sounds from the air, it must receive at least sixteen equally distant impulses or strokes in a second. The most common way of producing this effect is by a string or wire A B, Fig. 40, stretched between the fixed points A, B. If this string is taken by the middle and pulled aside, or if it is suddenly struck, it will vibrate between its two fixed points, as shown in the figure, passing alternately on each side of its axis A B, the vibrations gradually diminishing by the resistance of the air till the string is brought to rest. Its vibrations, however, may be kept up, by drawing a rosined fiddle-bow across it, and while it is181 vibrating it will give out a sound corresponding to the rapidity of its vibrations, and arising from the successive blows or impulses given to the air by the string. This sound is called the fundamental sound of the string, and its acuteness or sharpness increases with the number of vibrations which the string performs in a second.

Fig. 40.

If we now touch the vibrating string A´ B lightly with the finger, or with a feather at the middle point C, Fig. 40, it will give out a more acute but fainter sound than before, and while the extent of its vibrations is diminished, their frequency is doubled. In like manner, if we touch the string A´´ B´´, Fig. 40, at a point C, so that A´´ C is one-third of A´´ B´´, the note will be still more acute, and correspond to thrice the number of vibrations. All this might have been expected; but the wonderful part of the experiment is, that the vibrating string A´ B´ divides itself at C into two parts A´ C, C B´, the part A´ C vibrating round A and C as fixed points, and the part C B´ round C and B´, but always so that the part A´ C is at the same distance on the one side182 of the axis A´ B´ as at A m C, while the part C B is on the other side, as at C n B. Hence the point C, being always pulled by equal and opposite forces, remains at rest as if it were absolutely fixed. This stationary point is called a node, and the vibrating portions A´m C, C n B´ loops. The very same is true of the string A´´ B´´, the points C and D being stationary points; and upon the same principle a string may be divided into any number of vibrating portions. In order to prove that the string is actually vibrating in these equal subdivisions, we have only to place a piece of light paper with a notch in it on different parts of the string. At the nodes C and D it will remain perfectly at rest, while at m or n in the middle of the loops it will be thrown off or violently agitated.

The acute sounds given out by each of the vibrating portions are called harmonic sounds, and they accompany the fundamental sound of the string in the very same manner as we have already seen that the eye sees the accidental or harmonic colours while it is affected with the fundamental colour.

The subdivision of the string, and consequently the production of harmonic sounds, may be effected without touching the string at all, and by means of a sympathetic action conveyed by the air. If a string A B, for example, Fig. 40, is at rest, and if a shorter string A´´ C, one third of its length, fixed at the two points A´´ and C, is set vibrating in the same room, the string A B will be set vibrating in three loops like A´´ B´´, giving out the same harmonic sounds as the small string A´´ C.

183

It is owing to this property of sounding bodies that singers with great power of voice are able to break into pieces a large tumbler glass, by singing close to it its proper fundamental note; and it is from the same sympathetic communication of vibrations that two pendulum clocks fixed to the same wall, or two watches lying upon the same table, will take the same rate of going, though they would not agree with one another if placed in separate apartments. Mr. Ellicott even observed that the pendulum of the one clock will stop that of the other, and that the stopped pendulum will, after a certain time, resume its vibrations, and in its turn stop the vibrations of the other pendulum.

The production of musical sounds by the vibrations of a column of air in a pipe is familiar to every person, but the extraordinary mechanism by which it is effected is known principally to philosophers. A column of air in a pipe may be set vibrating by blowing over the open end of it, as is done in Pan’s pipes; or by blowing over a hole in its side, as in the flute; or by blowing through an aperture called a reed, with a flexible tongue, as in the clarionet. In order to understand the nature of this vibration, let AB, Fig. 41, be a pipe or tube, and let us place in it a spiral spring AB, in which the coil or spire are at equal distances, each end of the spiral being fixed to the end of the tube. This elastic spring may be supposed to represent the air in the pipe, which is of equal density throughout. If we take hold of the spring at m, and push the point m towards A and towards B in succession, it will give us a good idea of the vibration of an elastic column of air. When m is pushed towards A,184 the spiral spring will be compressed or condensed, as shown at m A, No. 2, while at the other end it will be dilated or rarefied, as shown at m B, and in the middle of the tube it will have the same degree of compression as in No. 1. When the string is drawn to the other end of the tube B, the spring will be, as in No. 3, condensed at the end B, and dilated at the end A. Now when a column of air vibrates in a pipe AB, the whole of it rushes alternately from B to A, as in No. 2, and from A to B as in No. 3, being condensed at the end A, No. 2, and dilated or rarefied at the end B, while in No. 3 it is rarefied at A and condensed at B, preserving its natural density at the middle point between A and B. In the case of the spring the ends AB are alternately pushed outwards and pulled inwards by the spring, the end A being pushed outwards in No. 2, and B pulled inwards, while in No. 3 A is pulled inwards and B pushed outwards.

Fig. 41.

That the air vibrating in a pipe is actually in the state now described, may be shown by boring small holes in the pipe, and putting over them pieces of a fine membrane. The membrane opposite to the middle part between A and B185 where the particles of the air have the greatest motion, will be violently agitated, while at points nearer the ends A and B it will be less and less affected.

Fig. 42.

Let us now suppose two pipes, AB, BC, to be joined together as in Fig. 42, and to be separated by a fixed partition at B; and let a spiral spring be fixed in each. Let the spring AB be now pushed to the end A, while the spring BC is pushed to C, as in No. 1, and back again, as in No. 2, but always in opposite directions; then it is obvious that the partition B is in No. 1 drawn in opposite directions towards A and towards C, and always with forces equal to each other: that is, when B is drawn slightly towards A, which it is at the beginning of the motion, it is also drawn slightly towards C; and when it is drawn forcibly towards A, as it is at the end of the motion of the spring, it is also drawn forcibly towards C. If the partition B, therefore, is moveable, it will still remain fixed during the opposite excursions of the spiral springs; nay, if we remove the partition, and hook the end of one spiral spring to the end of the other, the node or point of junction will remain stationary during the movements of the springs, because at every instant that point is drawn by equal and opposite forces.186 If three, four, or five spiral springs are joined in a similar manner, we may conceive them all vibrating between their nodes in the same manner.

Upon the very same principles we may conceive a long column of air without partitions dividing itself into two, three, or four smaller columns, each of which will vibrate between its nodes in the same manner as the spiral spring. At the middle point of each small vibrating column, the air will be of its natural density, like that of the atmosphere; while at the nodes B, &c. it will be in a state of condensation and rarefaction alternately.

If, when the air is vibrating in one column in the pipe AB, as in Fig. 41, No. 2, 3, we conceive a hole made in the middle, the atmospheric air will not rush in to disturb the vibration, because the air within the pipe and without it has exactly the same density. Nay, if, instead of a single hole, we were to cut a ring out of the pipe at the middle point, the column would vibrate as before. But if we bore a hole between the middle and one of the ends, where the vibrating column must be either in a state of condensation or rarefaction, the air must either rush out or rush in, in order to establish the equilibrium. The air opposite the hole will then be brought to the state of the external air, like that in the middle of the pipe; it will become the middle of a vibrating column: and the whole column of air, instead of vibrating as one, will vibrate as two columns, each column vibrating with twice the velocity, and yielding harmonic sounds along with the fundamental sound of the whole columns, in the same manner as we have already explained187 with regard to vibrating strings. By opening other holes we may subdivide a vibrating column into any number of smaller vibrating columns. The holes in flutes, clarionets, &c. are made for this purpose. When they are all closed up, the air vibrates in one column; and by opening and shutting the different holes in succession, the number of vibrating columns is increased or diminished at pleasure, and consequently the harmonic sounds will vary in a similar manner.

Curious as these phenomena are, they are still surpassed by those which are exhibited during the vibration of solid bodies. A rod or bar of metal or glass may be made to vibrate either longitudinally or laterally.

An iron rod will vibrate longitudinally, like a column of air, if we strike it at one end in the direction of its length; or rub it in the same direction with a wetted finger, and it will admit the same fundamental note as a column of air ten or eleven times as long, because sound moves so much faster in iron than in air. When the iron rod is thus vibrating along its length, the very same changes which we have shown in Fig. 41, as produced in a spiral spring, or in a column of air, take place in the solid metal. All its particles move alternately towards A and towards B, the metal being in the one case condensed at the end to which the particles move, and expanded at the end from which they move, and retaining its natural density in the middle of the rod. If we now hold this rod in the middle, by the finger and thumb lightly applied, and rub it in the middle either of AB or BC with a piece of cloth sprinkled with powdered rosin, or with a 188well-rosined fiddle-bow drawn across the rod, it will divide itself into two vibrating portions AB, BC, each of which will vibrate, as shown in Fig. 42, like the two adjacent columns of air, the section of the rod, or the particles which compose that section at B, being at perfect rest. By holding the rod at any intermediate point between A and B, so that the distance from A to the finger and thumb is one-third, one-fourth, one-fifth, &c. of the whole length AC, and rubbing one of the divisions in the middle, the rod will divide itself into 3, 4, 5, &c. vibrating portions, and give out corresponding harmonic sounds.

Fig. 43.

A rod of iron may be made to vibrate laterally or transversely, by fixing one end of it firmly, as in a vice, and leaving the other free, or by having both ends free or both fixed. When a rod, fixed at one end and free at the other, is made to vibrate, its mode of vibrating may be rendered evident to the eye; and for the purpose of doing this, Mr. Wheatstone has contrived a189 curious instrument, called the Kaleidophone, which is shown in Fig. 43. It consists of a circular base of wood AB, about nine inches in diameter and one inch thick, and having four brass sockets firmly fixed into it at C, D, E, and F. Into these sockets are screwed four vertical steel rods C, D, E, and F, about thirteen or fourteen inches long; one being a square rod, another a bent cylindrical190 one, and the other two cylindrical ones of different diameters. On the extremities of these rods are fixed small quicksilvered glass beads, either singly or in groups, so that when the instrument is placed in the light of the sun or in that of a lamp, bright images of the sun or flame are seen reflected on each bead. If any of these rods is set vibrating, these luminous images will form continuous and returning curve lines in a state of constant variation, each different rod giving curves of different characters, as shown in Fig. 44.

Fig. 44.

The Melodion, an instrument of great power, embracing five octaves, operates by means of the vibrations of metallic rods of unequal lengths, fixed at one end and free at the other.19 A narrow and thin plate of copper is screwed to the free extremity of each rod, and at right angles to its length; and its surface is covered with a small piece of felt, impregnated with rosin. This narrow band is placed near the circumference of a revolving cylinder, and, by touching the key, it is made to descend till it touches the revolving cylinder, and gives out its sound. The sweetness and power of this instrument are unrivalled; and such is the character of its tones, that persons of a nervous temperament are often entirely overpowered by its effects.

The vibrations of plates of metal or glass of various forms exhibit a series of the most extraordinary phenomena, which are capable of being shown by very simple means. These phenomena are displayed in an infinite variety of regular191 figures assumed by sand or fine lycopodium powder, strewed over the surface of the glass plate. In order to produce these figures, we must pinch or damp the plate at one or more places, and when the sand is strewed upon its surface, it is thrown into vibrations by drawing a fiddle-bow over different parts of its circumference. The method of damping or pinching plates is shown in Fig. 45. In No. 1, a square plate of glass AB, ground smooth at its edges, is pinched by the finger and thumb. In No. 2, a circular plate is held by the thumb against the top c of a perpendicular rod, and damped by the fingers at two different points of its circumference. In No. 3 it is damped at three points of its circumference; c and d by the thumb and finger, and at e by pressing it against a fixed obstacle a b. By means of a clamp like that at No. 4, it may be damped at a greater number of points.

Fig. 45.

192

Fig. 46.

If we take a square plate of glass, such as that shown in Fig. 46, No. 1, and, pinching it at its centre, draw the fiddle-bow near one of its angles, the sand will accumulate in the form of a cross, as shown in the figure, being thrown off the parts of the plate that are in a state of vibration, and settling in the nodes or parts which are at rest. If the bow is drawn across the middle of one of the edges, the sand will accumulate as in No. 2. If the plate is pinched at N, No. 3, and the bow applied at F and perpendicular to AB, the sand will arrange itself in three parallel lines, perpendicular to a fourth passing through F and N. But if the point N, where it is pinched, is a little farther from the edge than in No. 3, the parallel lines will change into curves as in No. 4.

If the plate of glass is circular, and pinched at its centre, and also at a point of its circumference,193 and if the bow is applied at a point 45° from the last point, the figure of the sand will be as in Fig. 47, No. 1. If with the same plate, similarly pinched, the bow is drawn over a part 30° from the pinched point of the circumference, the sand will form six radii as in No. 2. When the centre of the plate is left free, a different set of figures is produced, as shown in No. 3 and No. 4. When the plate is pinched near its edge, and the bow applied 45° from the point pinched, a circle of sand will pass through that point, and two diameters of sand, at right angles to each other, will be formed as in No. 3. When a point of the circumference is pressed against a fixed obstacle, and the bow applied 30° from that point, the figure in No. 4 is produced.

Fig. 47.

If, in place of a solid plate, we strew the sand194 over a stretched membrane, the sand will form itself into figures, even when the vibrations are communicated to the membrane through the air. In order to make these experiments, we must stretch a thin sheet of wet paper, such as vegetable paper, over the mouth of a tumbler-glass with a footstalk, and fix it to the edges with glue. When the paper is dry, a thin layer of dry sand is strewed upon its surface. If we place this membrane upon a table, and hold immediately above it, and parallel to the membrane, a plate of glass vibrating so as to give any of the figures shown in Fig. 47, the sand upon the membrane will imitate exactly the figure upon the glass. If the glass plate, in place of vibrating horizontally, is made to vibrate in an inclined position, the figures on the membrane will change with the inclination, and the sand will assume the most curious arrangements. The figures thus produced vary with the size of the membrane, with its material, its tension, and its shape. When the same figure occurs several times in succession, a breath upon the paper will change its degree of tension, and produce an entirely new figure, which, as the temporary moisture evaporates, will return to the original figure, through a number of intermediate ones. The pipe of an organ at the distance of a few feet, or the notes of a flute at the distance of half a foot, will arrange the sand on the membrane into figures which perpetually change with the sound that is produced.

The manner in which flat rulers and cylinders of glass perform their vibrations is very remarkable. If a glass plate about twenty-seven inches long, six-tenths of an inch broad, and six hun195dredths of an inch thick, is held by the edges between the finger and thumb, and has its lower surface, near either end, rubbed with a piece of wet cloth, sand laid upon its upper surface will arrange itself in parallel lines at right angles to the length of the plate. If the place of these lines is marked with a dot of ink, and the other side of the glass ruler is turned upwards, and the ruler made to vibrate as before, the sand will now accumulate in lines intermediate between the former lines, so that the motions of one-half the thickness of the glass ruler are precisely the reverse of those of the corresponding parts of the other half.

As these singular phenomena have not yet been made available by the scientific conjuror, we must be satisfied with this brief notice of them; but there is still one property of sound, which has its analogy also in light, too remarkable to be passed without notice. This property has more of the marvellous in it than any result within the wide range of the sciences. Two loud sounds may be made to produce silence, and two strong lights may be made to produce darkness!

If two equal and similar strings, or the columns of air in two equal and similar pipes, perform exactly 100 vibrations in a second, they will produce each equal waves of sound, and these waves will conspire in generating an uninterrupted sound, double of either of the sounds, heard separately. If the two strings or the two columns of air are not in unison, but nearly so, as in the case where the one vibrates 100 and the other 101 times in a second, then at the first vibration the two sounds will form one of double the strength of either;196 but the one will gradually gain upon the other, till at the fiftieth vibration it has gained half a vibration on the other. At this instant the two sounds will destroy one another, and an interval of perfect silence will take place. The sound will instantly commence, and gradually increase till it becomes loudest at the hundredth vibration, where the two vibrations conspire in producing a sound double of either. An interval of silence will again occur at the 150th, 250th, 350th vibration, or every second, while a sound of double the strength of either will be heard at the 200th, 300th, and 400th vibration. When the unison is very defective, or when there is a great difference between the number of vibrations which the two strings or columns of air perform in a second, the successive sounds and intervals of silence resemble a rattle. With a powerful organ, the effect of this experiment is very fine, the repetition of the sounds wow—wow—wow—representing the double sound and the interval of silence which arise from the total extinction of the two separate sounds.

The phenomenon corresponding to this in the case of light is perhaps still more surprising. If a beam of red light issues from a luminous point, and falls upon the retina, we shall see distinctly the luminous object from which it proceeds; but if another pencil of red light issues from another luminous point, anyhow situated, provided the difference between its distance and that of the other luminous point from the point of the retina, on which the first beam fell, is the 258th thousandth part of an inch, or exactly twice, thrice, four times, &c., that distance; and if this second beam falls upon the same point of the retina, the197 one light will increase the intensity of the other, and the eye will see twice as much light as when it received only one of the beams separately. All this is nothing more than what might be expected from our ordinary experience. But if the difference in the distances of the two luminous points is only one-half of the 258th thousandth part of an inch, or 1½, 2½, 3½, 4½, times that distance, the one light will extinguish the other and produce absolute darkness. If the two luminous points are so situated, that the difference of their distances from the point of the retina is intermediate between 1 and 1½, or 2 and 2½, above the 258th thousandth part of an inch, the intensity of the effect which they produce will vary from absolute darkness to double the intensity of either light. At 1¼, 2¼, 3¼ times, &c., the 258th thousandth of an inch, the intensity of the two combined lights will be equal only to one of them acting singly. If the lights, in place of falling upon the retina, fall upon a sheet of white paper, the very same effect will be produced, a black spot being produced in the one case, and a bright white one in the other, and intermediate degrees of brightness in intermediate cases. If the two lights are violet, the difference of distances at which the preceding phenomena will be produced will be the 157th thousandth part of an inch, and it will be intermediate between the 258th and the 157th thousandth part of an inch for the intermediate colours. This curious phenomenon may be easily shown to the eye, by admitting the sun’s light into a dark room through a small hole about the 40th or 50th part of an inch in diameter, and198 receiving the light on a sheet of paper. If we hold a needle or piece of slender wire in this light, and examine its shadow, we shall find that the shadow consists of bright and dark stripes succeeding each other alternately, the stripe in the very middle or axis of the shadow being a bright one. The rays of light which are bent into the shadow, and which meet in the very middle of the shadow, have exactly the same length of path, so that they form a bright fringe of double the intensity of either; but the rays which fall upon a point of the shadow at a certain distance from the middle, have a difference in the length of their paths, corresponding to the difference at which the lights destroy each other, so that a black stripe is produced on each side of the middle bright one. At a greater distance from the middle, the difference becomes such as to produce a bright stripe, and so on, a bright and a dark stripe succeeding each other to the margin of the shadow.

The explanation which philosophers have given of these strange phenomena is very satisfactory, and may be easily understood. When a wave is made on the surface of a still pool of water, by plunging a stone into it, the wave advances along the surface, while the water itself is never carried forward, but merely rises into a height and falls into a hollow, each portion of the surface experiencing an elevation and a depression in its turn. If we suppose two waves equal and similar to be produced by two separate stones, and if they reach the same spot at the same time, that is, if the two elevations should exactly coincide, they would unite their effects, and produce a199 wave twice the size of either; but if the one wave should be just so far before the other, that the hollow of the one coincided with the elevation of the other, and the elevation of the one with the hollow of the other, the two waves would obliterate or destroy one another, the elevation as it were of the one filling up half the hollow of the other, and the hollow of the one taking away half the elevation of the other, so as to reduce the surface to a level. These effects will be actually exhibited by throwing two equal stones into a pool of water, and it will be seen that there are certain lines of a hyperbolic form where the water is quite smooth, in consequence of the equal waves obliterating one another, while, in other adjacent parts, the water is raised to a height corresponding to both the waves united.

In the tides of the ocean we have a fine example of the same principle. The two immense waves arising from the action of the sun and moon upon the ocean produce our spring-tides by their combination, or when the elevations of each coincide; and our neap-tides, when the elevation of the one wave coincides with the depression of the other. If the sun and moon had exerted exactly the same force upon the ocean, or produced tide waves of the same size, then our neap-tides would have disappeared altogether, and the spring-tide would have been a wave double of the wave produced by the sun and moon separately. An example of the effect of the equality of the two waves occurs in the port of Batsha, where the two waves arrive by channels of different lengths, and actually obliterate each other.

Now, as sound is produced by undulations or waves in the air, and as light is supposed to be200 produced by waves or undulations in an ethereal medium, filling all nature, and occupying the pores of transparent bodies, the successive production of sound and silence by two loud sounds, or of light and darkness by two bright lights, may be explained in the very same manner as we have explained the increase and the obliteration of waves formed on the surface of water. If this theory of light be correct, then the breadth of a wave of red light will be the 258th thousandth part of an inch, the breadth of a wave of green light the 207th thousandth part of an inch, and the breadth of a wave of violet light the 157th thousandth part of an inch.

Among the wonders of modern skill, we must enumerate those beautiful automata by which the motions and actions of man and other animals have been successfully imitated. I shall therefore describe at present some of the most remarkable acoustic automata, in which the production of musical and vocal sounds has been the principal object of the artist.

Many very ingenious pieces of acoustic mechanism have been from time to time exhibited in Europe. The celebrated Swiss mechanist, M. le Droz, constructed for the King of Spain the figure of a sheep, which imitated in the most perfect manner the bleating of that animal; and likewise the figure of a dog watching a basket of fruit, which, when any of the fruit was taken away, never ceased barking till it was replaced.

The singing-bird of M. Maillardet, which he exhibited in Edinburgh many years ago, is still more wonderful.20 An oval box, about three201 inches long, was set upon the table, and in an instant the lid flew up, and a bird of the size of the humming-bird, and of the most beautiful plumage, started from its nest. After fluttering its wings, it opened its bill and performed four different kinds of the most beautiful warbling. It then darted down into its nest, and the lid closed upon it. The moving power in this piece of mechanism is said to have been springs which continued their action only four minutes. As there was no room within so small a figure for accommodating pipes to produce the great variety of notes which were warbled, the artist used only one tube, and produced all the variety of sounds by shortening and lengthening it with a moveable piston.

Ingenious as these pieces of mechanism are, they sink into insignificance when compared with the machinery of M. Vaucanson, which had previously astonished all Europe. His two principal automata were the flute-player, and the pipe and tabor-player. The flute-player was completed in 1736, and wherever it was exhibited it produced the greatest sensation. When it came to Paris it was received with great suspicion. The French savants recollected the story of M. Raisin, the organist of Troyes, who exhibited an automaton player upon the harpsichord, which astonished the French court by the variety of its powers. The curiosity of the king could not be restrained, and in consequence of his insisting upon examining the mechanism, there was found in the figure a pretty little musician five years of age. It was natural, therefore, that a similar piece of mechanism should be received with some distrust; but this feeling was soon removed by M. Vaucanson,202 who exhibited and explained to a committee of the Academy of Sciences the whole of the mechanism. This learned body was astonished at the ingenuity which it displayed; and they did not hesitate to state, that the machinery employed for producing the sounds of the flute performed in the most exact manner the very operations of the most expert flute-player, and that the artist had imitated the effects produced, and the means employed by nature, with an accuracy which exceeded all expectation. In 1738, M. Vaucanson published a memoir, approved of by the Academy, in which he gave a full description of the machinery employed, and of the principles of its construction. Following this memoir, I shall therefore attempt to give as popular a description of the automaton as can be done without lengthened details and numerous figures.

The body of the flute-player was about 5½ feet high, and was placed upon a piece of rock, surrounding a square pedestal 4½ feet high by 3½ feet wide. When the panel which formed the front of the pedestal was opened, there was seen on the right a clock movement, which, by the aid of several wheels, gave a rotatory motion to a steel axis about 2½ feet long, having cranks at six equidistant points of its length, but lying in different directions. To each crank was attached a cord, which descended and was fixed by its other end to the upper board of a pair of bellows, 2½ feet long and 6 inches wide. Six pair of bellows arranged along the bottom of the pedestal were then wrought, or made to blow in succession, by turning the steel axis.

At the upper face of the pedestal, and upon203 each pair of bellows is a double pulley, one of whose rims is 3 inches in diameter, and the other 1½. The cord which proceeds from the crank coils round the smaller of these pulleys, and that which is fixed to the upper board of the bellows goes round the larger pulley. By this means the upper board of the bellows is made to rise higher than if the cords went directly from them to the cranks.

Round the larger rims of three of these pulleys, viz. those on the right hand, there are coiled three cords, which, by means of several smaller pulleys, terminate in the upper boards of other three pair of bellows placed on the top of the box.

The tension of each cord when it begins to raise the board of the bellows to which it is attached, gives motion to a lever placed above it between the axis and the double pulley in the middle and lower region of the box. The other end of this lever keeps open the valve in the lower board of the bellows, and allows the air to enter freely, while the upper board is rising to increase the capacity of the bellows. By this means there is not only power gained, in so far as the air gains easier admission through the valve, but the fluttering noise produced by the action of the air upon the valves is entirely avoided, and the nine pair of bellows are wrought with great ease, and without any concussion or noise.

These nine bellows discharge their wind into three different and separate tubes. Each tube receives the wind of three bellows, the upper boards of one of the three pair being loaded with a weight of four pounds, those of the second three pair with a weight of two pounds,204 and those of the other three pair with no weight at all. These three tubes ascended through the body of the figure and terminated in three small reservoirs placed in its trunk. These reservoirs were thus united into one, which, ascending into the throat, formed by its enlargement the cavity of the mouth terminated by two small lips, which rested upon the whole of the flute. These lips had the power of opening more or less, and by a particular mechanism, they could advance or recede from the hole in the flute. Within the cavity of the mouth there is a small moveable tongue for opening and shutting the passage for the wind through the lips of the figure.

The motions of the fingers, lips, and tongue of the figure were produced by means of a revolving cylinder, thirty inches long and twenty-one in diameter. By means of pegs and brass staples fixed in fifteen different divisions in its circumference, fifteen different levers, similar to those in a barrel organ, were raised and depressed. Seven of these regulated the motions of the seven fingers for stopping the holes of the flute, which they did by means of steel chains rising through the body, and directed by pulleys to the shoulder, elbow, and fingers. Other three of the levers communicating with the valves of the three reservoirs, regulated the ingress of the air, so as to produce a stronger or a weaker tone. Another lever opened the lips so as to give a free passage to the air, and another contracted them for the opposite purpose. A third lever drew them backwards from the orifice of the flute, and a fourth pushed them forward. The remaining205 lever enabled the tongue to stop up the orifice of the flute.

Such is a very brief view of the general mechanism by which the requisite motions of the flute-player were produced. The airs which it played were probably equal to those executed by a living performer, and its construction, as well as its performances, continued for many years to delight and astonish the philosophers and musicians of Europe.

Encouraged by the success of this machine, M. Vaucanson exhibited in 1741 other automata, which were equally, if not more, admired. One of these was the automaton duck, which performed all the motions of that animal, and not only ate its food, but digested it;21 and the other was his pipe and tabor-player, a piece of mechanism which required all the resources of his fertile genius. Having begun this machine before he was aware of its peculiar difficulties, he was often about to abandon it in despair, but his patience and his ingenuity combined, enabled him not only to surmount every difficulty, but to construct an automaton which performed complete airs, and greatly excelled the most esteemed performers on the pipe and tabor.

The figure stands on a pedestal, and is dressed like a dancing shepherd. He holds in one hand a flageolet, and in the other the stick with which he beats the tambourine as an accompaniment to the airs of the flageolet, about twenty of which it is capable of performing. The flageolet has only three holes, and the variety of its tones depends principally on a proper variation of the force of206 the wind, and on the different degrees with which the orifices are covered. These variations in the force of the wind required to be given with a rapidity which the ear can scarcely follow, and the articulation of the tongue was required for the quickest notes, otherwise the effect was far from agreeable. As the human tongue is not capable of giving the requisite articulations to a rapid succession of notes, and generally slurs over one-half of them, the automaton was thus able to excel the best performers, as it played complete airs with articulations of the tongue at every note.

In constructing this machine, M. Vaucanson observed that the flageolet must be a most fatiguing instrument for the human lungs, as the muscles of the chest must make an effort equal to fifty-six pounds in order to produce the highest notes. A single ounce was sufficient for the lowest notes: so that we may, from this circumstance, form an idea of the variety of intermediate effects required to be produced.

While M. Vaucanson was engaged in the construction of these wonderful machines, his mind was filled with the strange idea of constructing an automaton containing the whole mechanism of the circulation of the blood. From some birds which he made, he was satisfied of its practicability; but as the whole vascular system required to be made of elastic gum or caoutchouc, it was supposed that it could only be executed in the country where the caoutchouc tree was indigenous. Louis XVI. took a deep interest in the execution of this machine. It was agreed that a skilful anatomist should proceed to Guiana to super207intend the construction of the blood-vessels, and the king had not only approved of, but had given orders for, the voyage. Difficulties, however, were thrown in the way, Vaucanson became disgusted, and the scheme was abandoned.

The two automata which we have described were purchased by Professor Bayreuss of Helmstadt; but we have not been able to learn whether or not they still exist.

Fig. 48.

Towards the end of the eighteenth century a bold and almost successful attempt was made to construct a talking automaton. In the year 1779, the Imperial Academy of Sciences at St. Petersburgh proposed, as the subject of one of their annual prizes, an inquiry into the nature of the vowel sounds, A, E, I, O, and U, and the construction of an instrument for artificially imitating them. This prize was gained by M. Kratzenstein, who showed that all the vowels could be distinctly pronounced by blowing through a reed into the lower ends of the pipes of the annexed figures, as shown in Fig. 48, where the corresponding vowels are marked on the different pipes. The vowel I is pronounced by merely blowing208 into the pipe a b, of the pipe marked I, without the use of a reed.

Fig. 49.

About the same time that Kratzenstein was engaged in these researches, M. Kempelen of Vienna, a celebrated mechanician, was occupied with the same subject. In his first attempt he produced the vowel sounds, by adapting a reed R, Fig. 49, to the bottom of a funnel-shaped cavity A B, and placing his hand in various positions within the funnel. This contrivance, however, was not fitted for his purpose, but after long study, and a diligent examination of the organs of speech, he contrived a hollow oval box, divided into two portions attached by a hinge so as to resemble jaws. This box received the sound which issued from the tube connected with the reed, and by opening and closing the jaws, he produced the sounds, A, O, OU, and an imperfect E, but no indications of an I. After two years’ labour he succeeded in obtaining from different jaws the sounds of the consonants P, M, L, and by means of these vowels and consonants, he could compose syllables and words, such as mama, papa, aula, lama, mulo. The209 sounds of two adjacent letters, however, ran into each other, and an aspiration followed some of the consonants; so that, instead of papa, the word sounded phaa-ph-a; these difficulties he contrived with much labour to surmount, and he found it necessary to imitate the human organs of speech by having only one mouth and one glottis. The mouth consisted of a funnel, or bell-shaped piece of elastic gum, which approximated, by its physical properties, to the softness and flexibility of the human organs.22 To the mouth-piece was added a nose made of two tin tubes, which communicated with the mouth. When both these tubes were open, and the mouth-piece closed, a perfect M was produced; and when one was closed and the other open, an N was sounded. M. Kempelen could have succeeded in obtaining the four letters D, G, K, T, but, by using a P instead of them, and modifying the sound in a particular manner, he contrived to deceive the ear by a tolerable resemblance of these letters.

There seems to be no doubt that he at last was able to produce entire words and sentences, such as opera, astronomy, Constantinopolis, Vous êtes mon ami, Je vous aime de tout mon cœur, Venez avec moi à Paris, Leopoldus secundus, Romanorum imperator semper Augustus, &c., but he never fitted up a speaking figure; and probably, from being dissatisfied with the general result210 of his labours, he exhibited only to his private friends the effects of the apparatus, which was fitted up in the form of a box.

This box was rectangular, and about three feet long, and was placed upon a table, and covered with a cloth. When any particular word was mentioned by the company, M. Kempelen caused the machine to pronounce it, by introducing his hands beneath the cloth, and apparently giving motion to some parts of the apparatus. Mr. Thomas Collinson, who had seen this machine in London, mentions, in a letter to Dr. Hutton, that he afterwards saw it at M. Kempelen’s own house in Vienna, and that he then gave it the same word to be pronounced which he gave it in London, viz. the word Exploitation, which, he assures us, it again distinctly pronounced with the French accent.

M. Kratzenstein seems to have been equally unsuccessful; for though he assured M. de Lalande, when he saw him in Paris, in 1786, that he had made a machine which could speak pretty well, and though he showed him some of the apparatus by which it could sound the vowels, and even such syllables as papa and mama, yet there is no reason to believe that he had accomplished more than this.

The labours of Kratzenstein and Kempelen have been recently pursued with great success by our ingenious countryman, Mr. Willis, of Cambridge. In repeating Kempelen’s experiment, shown in Fig. 49, he used a shallower cavity, such as that in Fig. 50, and found that he could entirely dispense with the introduction of the hand, and could obtain the whole series of211 vowels by sliding a flat board C D over the mouth of the cavity. Mr. Willis then conceived the idea of adapting to the reed cylindrical tubes, whose length could be varied by sliding joints. When the tube was greatly less than the length of a stopped pipe in unison with the reed, it sounded I, and by increasing the length of the tube, it gave E, A, O, and U, in succession. But what was very unexpected, when the tube was so much lengthened as to be 1½ times the length of a stopped pipe in unison with the reed, the vowels began to be again sounded in an inverted order, viz. U, O, A, E, and then again in a direct order, I, E, A, O, U, when the length of the tube was equal to twice that of a stopped pipe, in unison with the reed.

Fig. 50.

Some important discoveries have been recently made by M. Savart respecting the mechanism of the human voice;23 and we have no doubt that, before another century is completed, a Talking and a Singing machine will be numbered among the conquests of Science.

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LETTER IX.

Singular effects in nature depending on sound—Permanent character of speech—Influence of great elevations on the character of sounds, and on the powers of speech—Power of sound in throwing down buildings—Dog killed by sound—Sounds greatly changed under particular circumstances—Great audibility of sounds during the night explained—Sounds deadened in media of different densities—Illustrated in the case of a glass of champagne—and in that of new-fallen snow—Remarkable echoes—Reverberations of thunder—Subterranean noises—Remarkable one at the Solfaterra—Echo at the Menai suspension bridge—Temporary deafness produced in diving-bells—Inaudibility of particular sounds to particular ears—Vocal powers of the statue of Memnon—Sounds in granite rocks—Musical mountain of El-Nakous.

Although, among the phenomena of the material world, there is scarcely one which, when well considered, is not an object of wonder, yet those which we have been accustomed to witness from our infancy lose all their interest from the frequency of their occurrence, while to the natives of other countries they are unceasing objects of astonishment and delight. The inhabitant of a tropical climate is confounded at the sight of falling snow, and he almost discredits the evidence of his senses when he sees a frozen river carrying loaded waggons on its surface. The diffusion of knowledge by books, as well as by frequent communication between the natives of213 different quarters of the globe, has deprived this class of local wonders of their influence, and the Indian and the Scandinavian can visit each other’s lands without any violent excitement of surprise. Still, however, there are phenomena of rare occurrence, of which no description can convey the idea, and which continue to be as deeply marked with the marvellous as if they had been previously unknown. Among these we may rank the remarkable modifications which sound undergoes in particular situations and under particular circumstances.

In the ordinary intercourse of life, we recognize individuals as much by their voice as by the features of their face and the form of their body. A friend who has been long absent will often stand before us as a stranger, till his voice supplies us with the full power of recognition. The brand imprinted by time on his outer form may have effaced the youthful image which the memory had cherished, but the original character of his voice and its yet remembered tones will remain unimpaired.

An old friend with a new face is not more common in its moral than in its physical acceptation; and though the sagacity of proverbial wisdom has not supplied us with the counterpart in relation to the human voice, yet the influence of its immutability over the mind has been recorded by the poet in some of his most powerful conceptions. When Manfred was unable to recognize in the hectic phantom of Astarte the endeared lineaments of the being whom he loved, the mere utterance of his name recalled “the214 voice which was his music,” and invested her with the desired reality.

The permanence of character thus impressed upon speech exists only in those regions to whose atmosphere our vocal organs are adapted. If either the speaker or the hearer is placed in air differing greatly in density from that to which they are accustomed, the voice of the one will emit different sounds, or the same sounds will produce a different impression on the ear of the other. But if both parties are placed in this new atmosphere, their tones of communication will suffer the most remarkable change. The two extreme positions, where such effects become sufficiently striking, are in the compressed air of the diving-bell, when it is immersed to a great depth in the sea, or in the rarefied atmosphere which prevails on the summit of the Himalaya or the Andes.

In the region of common life, and even at the stillest hour of night, the ear seldom rests from its toils. When the voice of man and the bustle of his labours have ceased, the sounds of insect life are redoubled; the night breeze awakens among the rustling leaves, and the swell of the distant ocean, and the sounds of the falling cataract or of the murmuring brook, fill the air with their pure and solemn music. The sublimity of deep silence is not to be found even in the steppes of the Volga, or in the forests of the Orinoco. It can be felt only in those lofty regions

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As the traveller rises above the limit of life and motion, and enters the region of habitual solitude, the death-like silence which prevails around him is rendered still more striking by the diminished density of the air which he breathes. The voice of his fellow traveller ceases to be heard even at a moderate distance, and sounds which would stun the ear at a lower level make but a feeble impression. The report of a pistol on the top of Mont Blanc is no louder than that of an Indian cracker. But while the thinness of the air thus subdues the loudest sounds, the voice itself undergoes a singular change: the muscular energy by which we speak experiences a great diminution, and our powers of utterance, as well as our power of hearing, are thus singularly modified. Were the magician, therefore, who is desirous to impress upon his victim or upon his pupil the conviction of his supernatural power, to carry him, under the injunction of silence,

he would experience little difficulty in asserting his power over the elements, and still less in subsequently communicating the same influence to his companion.

But though the air at the tops of our highest mountains is scarcely capable of transmitting sounds of ordinary intensity, yet sounds of extraordinary power force their way through its most attenuated strata. At elevations where the air is three thousand times more rare than that which we breathe, the explosion of meteors is216 heard like the sound of cannon on the surface of the earth, and the whole air is often violently agitated by the sound. This fact alone may give us some idea of the tremendous nature of the forces which such explosions create, and it is fortunate for our species that they are confined to the upper regions of the atmosphere. If the same explosions were to take place in the dense air which rests upon the earth, our habitations and our lives would be exposed to the most imminent peril.

Buildings have often been thrown down by violent concussions of the air, occasioned either by the sound of great guns or by loud thunder, and the most serious effects upon human and animal life have been produced by the same cause. Most persons have experienced the stunning pain produced in the ear, when placed near a cannon that is discharged. Deafness has frequently been the result of such sudden concussions, and, if we may reason from analogy, death itself must often have been the consequence. When peace was proclaimed in London, in 1697, two troops of horse were dismounted and drawn up in line in order to fire their volleys. Opposite the centre of the line was the door of a butcher’s shop, where there was a large mastiff dog of great courage. This dog was sleeping by the fire, but when the first volley was fired, it immediately started up, ran into another room, and hid itself under a bed. On the firing of the second volley, the dog rose, ran several times about the room trembling violently, and apparently in great agony. When the third volley was fired, the dog ran about once or twice with217 great violence and instantly fell down dead, throwing up blood from his mouth and nose.

Sounds of known character and intensity are often singularly changed even at the surface of the earth, according to the state of the ground and the conditions of the clouds. On the extended heath, where there are no solid objects capable of reflecting or modifying sound, the sportsman must frequently have noticed the unaccountable variety of sounds which are produced by the report of his fowling-piece. Sometimes they are flat and prolonged, at other times short and sharp, and sometimes the noise is so strange that it is referred to some mistake in the loading of the gun. These variations, however, arise entirely from the state of the air, and from the nature and proximity of the superjacent clouds. In pure air of uniform density the sound is sharp and soon over, as the undulations of the air advance without any interrupting obstacles. In a foggy atmosphere, or where the vapours produced by heat are seen dancing as it were in the air, the sound is dull and prolonged; and when these clouds are immediately over-head, a succession of echoes from them produces a continued or reverberating sound. When the French astronomers were determining the velocity of sound by firing great guns, they observed that the report was always single and sharp under a perfectly clear sky, but indistinct, and attended by a long-continued roll like thunder, when a cloud covered a considerable part of the horizon. It is no doubt owing to the same cause, namely, the reflexion from the clouds, that the thunder rolls through the heavens, as if it were produced by a succession of electric explosions.

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The great audibility of sounds during the night is a phenomenon of considerable interest, and one which had been observed even by the ancients. In crowded cities or in their vicinity, the effect was generally ascribed to the rest of animated beings; while in localities where such an explanation was inapplicable, it was supposed to arise from a favourable direction of the prevailing wind. Baron Humboldt was particularly struck with this phenomenon when he first heard the rushing of the great cataracts of the Orinoco in the plain which surrounds the Mission of the Apures. These sounds he regarded as three times louder during the night than during the day. Some authors ascribed this fact to the cessation of the humming of insects, the singing of birds, and the action of the wind on the leaves of the trees, but M. Humboldt justly maintains that this cannot be the cause of it on the Orinoco, where the buzz of insects is much louder in the night than in the day, and where the breeze never rises till after sunset. Hence he was led to ascribe the phenomenon to the perfect transparency and uniform density of the air, which can exist only at night after the heat of the ground has been uniformly diffused through the atmosphere. When the rays of the sun have been beating on the ground during the day, currents of hot air of different temperatures, and consequently of different densities, are constantly ascending from the ground and mixing with the cold air above. The air thus ceases to be a homogeneous medium, and every person must have observed the effects of it upon objects seen through it which are very indistinctly visible, and have a tremulous motion, as if they were “dancing in the air.” The very219 same effect is perceived when we look at objects through spirits and water that are not perfectly mixed, or when we view distant objects over a red-hot poker or over a flame. In all these cases the light suffers refraction in passing from a medium of one density into a medium of a different density, and the refracted rays are constantly changing their direction as the different currents rise in succession. Analogous effects are produced when sound passes through a mixed medium, whether it consists of two different mediums or of one medium where portions of it have different densities. As sound moves with different velocities through media of different densities, the wave which produces the sound will be partly reflected in passing from one medium to the other, and the direction of the transmission wave changed: and hence in passing through such media different portions of the wave will reach the ear at different times, and thus destroy the sharpness and distinctness of the sound. This may be proved by many striking facts. If we put a bell in a receiver containing a mixture of hydrogen gas and atmospheric air, the sound of the bell can scarcely be heard. During a shower of rain or of snow, noises are greatly deadened; and when sound is transmitted along an iron wire or an iron pipe of sufficient length, we actually hear two sounds, one transmitted more rapidly through the solid, and the other more slowly through the air. The same property is well illustrated by an elegant and easily repeated experiment of Chladni’s. When sparkling champagne is poured into a tall glass till it is half full, the glass loses its power of ringing by a stroke220 upon its edge, and emits only a disagreeable and puffy sound. This effect will continue while the wine is filled with bubbles of air, or as long as the effervescence lasts; but when the effervescence begins to subside the sound becomes clearer and clearer, and the glass rings as usual when the air-bubbles have vanished. If we reproduce the effervescence by stirring the champagne with a piece of bread, the glass will again cease to ring. The same experiment will succeed with other effervescing fluids.

The difference in the audibility of sounds that pass over homogeneous and over mixed media is sometimes so remarkable as to astonish those who witness it. The following fact is given on the evidence of an officer who observed it:—When the British and the American forces were encamped on each side of a river, the outposts were so near, that the form of individuals could be easily distinguished. An American drummer made his appearance, and began to beat his drum; but though the motion of his arms was distinctly seen, not a single sound reached the ear of the observer. A coating of snow that had newly fallen upon the ground, and the thickness of the atmosphere, had conspired to obstruct the sound. An effect the very reverse of this is produced by a coating of glazed or hardened snow, or by an extended surface of ice or water. Lieutenant Foster was able to carry on a conversation with a sailor across Port Bowen Harbour, a distance of no less than a mile and a quarter, and the sound of great guns has been heard at distances varying from 120 to 200 miles. Over hard and dry ground of a uniform character, or221 where a thin soil rests upon a continuous stratum of rock, the sound is heard at a great distance, and hence it is the practice among many Eastern tribes to ascertain the approach of an enemy by applying the ear to the ground.

Many remarkable phenomena in the natural world are produced by the reflexion and concentration of sound. Every person is familiar with the ordinary echo which arises from the reflexion of sound from an even surface, such as the face of a wall, of a house, of a rock, of a hill, or of a cloud. As sound moves at the rate of 1090 feet in a second, and as the sound which returns to the person who emits it has travelled over a space equal to twice his distance from the reflecting surface, the distance in feet of the body which occasions the echo may be readily found by multiplying 545 by the number of seconds which elapse between the emission of the sound and its return in the form of an echo. This kind of echo, where the same person is the speaker and the hearer, never takes place, unless when the observer is immediately in front of the reflecting surface, or when a line drawn from his mouth to the flat surface is nearly perpendicular to it, because in this case alone the wave of sound is reflected in the very same direction from the wall in which it reaches it. If the speaker places himself on one side of this line, then the echo will be heard most distinctly by another person as far on the other side of it, because the waves of sound are reflected like light, so that the angle of incidence or the inclination at which the sound falls upon the reflected surface is equal to the angle of reflexion, or the inclination at which the222 sound is returned from the wall. If two persons, therefore, are placed before the reflecting wall, the one will hear the echo of the sound emitted by the other, and obstacles may intervene between these two persons, so that neither of them hears the direct sound emitted by the other; in the same manner as the same persons similarly placed before a looking-glass would see each other distinctly by reflexion, though objects might obstruct their direct view of each other.

Hitherto we have supposed that there is only one reflecting surface, in which case there will be only one echo; but if there are several reflecting surfaces, as in the case in an amphitheatre of mountains, or during a thunder-storm, where there are several strata or masses of clouds; or if there are two parallel or inclined surfaces between which the sound can be repeatedly reflected, or if the surface is curved, so that the sound reflected from one part falls upon another part, like the sides of a polygon inscribed in a circle,—in all these cases there will be numerous echoes, which produce a very singular effect. Nothing can be more grand and sublime than the primary and secondary echoes of a piece of ordnance discharged in an amphitheatre of precipitous mountains. The direct or primary echoes from each reflecting surface reach the ear in succession, according to their different distances, and these are either blended with or succeeded by the secondary echoes, which terminate in a prolonged growl, ending in absolute silence. Of the same character are the reverberated claps of a thunderbolt reflected from the surrounding clouds, and dying away in the distance. The223 echo which is produced by parallel walls is finely illustrated at the Marquis of Simonetta’s villa near Milan, which has been described by Addison and Keysler, and which we believe is that described by Mr. Southwell in the Philosophical Transactions for 1746. Perpendicular to the main body of this villa there extend two parallel wings about fifty-eight paces distant from each other, and the surfaces of which are unbroken either with doors or windows. The sound of the human voice, or rather a word quickly pronounced, is repeated above forty times, and the report of a pistol from fifty-six to sixty times. The repetitions, however, follow in such rapid succession that it is difficult to reckon them, unless early in the morning before the equal temperature of the atmosphere is disturbed, or in a calm, still evening. The echoes appear to be best heard from a window in the main building between the two projecting walls, from which the pistol also is fired. Dr. Plot mentions an echo in Woodstock Park which repeats seventeen syllables by day and twenty by night. An echo on the north side of Shipley church, in Sussex, repeats twenty-one syllables. Sir John Herschel mentions an echo in the Manfroni palace at Venice, where a person standing in the centre of a square room about twenty-five feet high, with a concave roof, hears the stamp of his foot repeated a great many times; but as his position deviates from the centre, the echoes become feebler, and at a short distance entirely cease. The same phenomenon, he remarks, occurs in the large room of the library of the museum at Naples. M. Genefay has described, as existing224 near Rouen, a curious oblique echo which is not heard by the person who emits the sound. A person who sings hears only his own voice, while those who listen hear only the echo, which sometimes seems to approach, and at other times seems to recede from the ear; one person hears a single sound, another several sounds, and one hears it on the right, and another on the left, the effect always changing as the hearer changes his position. Dr. Birch has described an extraordinary echo at Roseneath, in Argyleshire, which certainly does not now exist. When eight or ten notes were played upon a trumpet, they were correctly repeated, but on a key a third lower. After a short pause, another repetition of the notes was heard in a still lower tone, and after another short interval they were repeated in a still lower tone.

In the same manner as light is always lost by reflexion, so the waves of sound are enfeebled by reflexion from ordinary surfaces, and the echo is in such cases fainter than the original sound. If the reflecting surface, however, is circular, sound may be condensed and rendered stronger in the same manner as light. I have seen a fine example of this, in the circular turn of a garden wall nearly a mile distant from a weir across a river. When the air is pure and homogeneous, the rushing sound of the water is reflected from the hollow surface of the wall, and concentrated in a focus, the place of which the ear can easily discover from the intensity of the sound being there a maximum. A person not acquainted with the locality conceives that the rushing noise is on the other side of the wall.

In whispering galleries, or places where the225 lowest whispers are carried to distances at which the direct sound is inaudible, the sound may be conveyed in two ways, either by repeated reflexions from a curved surface in the direction of the sides of a polygon inscribed in a circle, or where the whisperer is in the focus of one reflecting surface, and the hearer in the focus of another reflecting surface, which is placed so as to receive the reflected sounds. The first of these ways is exemplified in the whispering gallery of St. Paul’s, and in the octagonal gallery of Gloucester cathedral, which conveys a whisper seventy-five feet across the nave; and the second in the baptistery of a church in Pisa, where the architect, Giovanni Pisano, is said to have constructed the cupola on purpose. The cupola has an elliptical form, and when one person whispers in one focus, it is distinctly heard by the person placed in the other focus, but not by those who are placed between them. The sound first reflected passes across the cupola, and enters the ears of the intermediate persons, but it is too feeble to be heard till it has been condensed by a second reflexion to the other focus of ellipse. A naval officer, who travelled through Sicily in the year 1824, gives an account of a powerful whispering place in the cathedral of Girgenti, where the slightest whisper is carried with perfect distinctness through a distance of two hundred and fifty feet, from the great western door to the cornice behind the high altar. By an unfortunate coincidence, the focus of one of the reflecting surfaces was chosen for the place of the confessional; and when this was accidentally discovered, the lovers of secrets resorted to the other focus, and thus226 became acquainted with confessions of the gravest import. This divulgence of scandal continued for a considerable time, till the eager curiosity of one of the dilettanti was punished, by hearing his wife’s avowal of her own infidelity. This circumstance gave publicity to the whispering peculiarity of the cathedral, and the confessional was removed to a place of greater secrecy.

Fig. 51.

An echo of a very peculiar character has been described by Sir John Herschell in his Treatise on Sound, as produced by the suspension bridge across the Menai strait in Wales. “The sound of a blow with a hammer,” says he, “on one of the main piers, is returned in succession from each of the cross-beams which support the road-way, and from the opposite pier at a distance of five hundred and seventy-six feet; and in addition to this, the sound is many times repeated between the water and the road-way. The effect is a series of sounds which may be thus written: the first return is sharp and strong from the road-way over-head; the rattling which succeeds dies away rapidly, but the single repercussion from the opposite pier is very strong, and is succeeded by a faint palpitation, repeating the sound at the rate of twenty-eight times in five seconds, and which, therefore corresponds to a distance of a hundred and eighty-four feet, or very nearly the double interval from the road-way to the227 water. Thus it appears that in the repercussion between the water and road-way, that from the latter only affects the ear, the line drawn from the auditor to the water being too oblique for the sound to diverge sufficiently in that direction. Another peculiarity deserves especial notice, namely, that the echo from the opposite pier is best heard when the auditor stands precisely opposite to the middle of the breadth of the pier, and strikes just on that point. As it deviates to one or the other side, the return is proportionally fainter, and is scarcely heard by him when his station is a little beyond the extreme edge of the pier, though another person, stationed (on the same side of the water) at an equal distance from the central point, so as to have the pier between them, hears it well.”

A remarkable subterranean echo is often heard when the hoofs of a horse or the wheels of a carriage pass over particular spots of ground. This sound is frequently very similar to that which is produced in passing over an arch or vault, and is commonly attributed to the existence of natural or artificial caves beneath. As such caves have often been constructed in times of war as places of security for persons and property, many unavailing attempts have been made to discover hidden treasures where their locality seemed to be indicated by subterraneous sounds. But though these sounds are sometimes produced by excavations in the ground, yet they generally arise from the nature of the materials of which the ground is composed, and from their manner of combination. If the hollow of a road has been filled up with broken rock, or with large water228worn stones, having hollows either left entirely empty, or filled up with materials of different density, then the sound will be reflected in passing from the loose to the dense materials, and there will arise a great number of echoes reaching the ear in rapid succession, and forming by their union a hollow rumbling sound. This principle has been very successfully applied by Sir John Herschell to explain the subterranean sounds with which every traveller is familiar who has visited the Solfaterra, near Naples. When the ground at a particular place is struck violently by throwing a large stone against it, a peculiar hollow sound is distinctly heard. This sound has been ascribed by some geologists to the existence of a great vault communicating with the ancient seat of the volcano, by other writers to a reverberation from the surrounding hills with which it is nearly concentric, and by others to the porosity of the ground. Dr. Daubeny, who says that the hollow sound is heard when any part of the Solfaterra is struck, accounts for it by supposing that the hill is not made up of one entire rock, but of a number of detached blocks, which, hanging as it were by each other, form a sort of vault over the abyss within which the volcanic operations are going on.24 Mr. Forbes, who has given the latest and most interesting description of this singular volcano,25 agrees in opinion with Dr. Daubeny; while Mr. Scrope26 and Sir John Herschell concur in opinion229 that no such cavities exist. “It seems most probable,” says the latter, “that the hollow reverberation is nothing more than an assemblage of partial echoes arising from the reflexion of successive portions of the original sound, in its progress through the soil at the innumerable half-coherent surfaces composing it: were the whole soil a mass of sand, these reflexions would be so strong and frequent as to destroy the whole impulse in too short an interval to allow of a distinguishable after-sound. It is a case analogous to that of a strong light thrown into a milky medium or smoky atmosphere; the whole medium appears to shine with a nebulous undefined light. This is to the eye what such a hollow sound is to the ear.”27

It has been recently shown by M. Savart, that the human ear is so extremely sensible as to be capable of appreciating sounds which arise from about twenty-four thousand vibrations in a second, and consequently that it can hear a sound which lasts only the twenty-four thousandth part of a second. Vibrations of such frequency afford only a shrill squeak or chirp; and Dr. Wollaston has shown that there are many individuals with their sense of hearing entire, who are altogether insensible to such acute sounds, though others are painfully affected by them. Nothing, as Sir John Herschell remarks, can be more surprising than to see two persons, neither of them deaf, the one complaining of the penetrating shrillness of a sound, while the other maintains there is no sound at all. Dr. Wollaston has also shown that this is true also of very grave sounds; so that230 the hearing or not hearing of musical notes at both extremities of the scale seems to depend wholly on the pitch or frequency of vibration constituting the note, and not upon the intensity or loudness of the noise. This affection of the ear sometimes appears in cases of common deafness, where a shrill tone of voice, such as that of women and children, is often better heard than the loud and deeper tone of men.

Dr. Wollaston remarked, that when the mouth and nose are shut, the tympanum or drum of the ear may be so exhausted by a forcible attempt to take breath by the expansion of the chest, the pressure of the external air upon the membrane gives it such a tension, that the ear becomes insensible to grave tones, without losing in any degree the perception of sharper sounds. Dr. Wollaston found, that after he had got into the habit of making the experiment, so as to be able to produce a great degree of exhaustion, his ears were insensible to all sounds below F, marked by the bass clef. “If I strike the table before me,” says he, “with the end of my finger, the whole board sounds with a deep dull note. If I strike it with my nail, there is also at the same time a sharp sound produced by quicker vibrations of parts around the point of contact. When the ear is exhausted, it hears only the latter sound, without perceiving in any degree the deeper note of the whole table. In the same manner, in listening to the sound of a carriage, the deeper rumbling noise of the body is no longer heard by an exhausted ear; but the rattle of a chain or loose screw remains at least as audible as before exhaustion.” Dr. Wollaston231 supposes that this excessive tension of the drum of the ear, when produced by the compressed air in the diving-bell, will also produce a corresponding deafness to low tones. This curious experiment has been since made by Dr. Colladon, when descending in the diving-bell at Howth, in 1820. “We descended,” says he, “so slowly that we did not notice the motion of the bell; but as soon as the bell was immersed in water, we felt about the ears and the forehead a sense of pressure, which continued increasing during some minutes. I did not, however, experience any pain in the ears; but my companion suffered so much that we were obliged to stop our descent for a short time. To remedy that inconvenience, the workmen instructed us, after having closed our nostrils and mouth, to endeavour to swallow, and to restrain our respiration for some moments, in order that, by this exertion, the internal air might act on the Eustachian tube. My companion, however, having tried it, found himself very little relieved by this remedy. After some minutes, we resumed our descent. My friend suffered considerably; he was pale; his lips were totally discoloured; his appearance was that of a man on the point of fainting; he was in involuntary low spirits, owing, perhaps, to the violence of the pain, added to that kind of apprehension which our situation unavoidably inspired. This appeared to me the more remarkable, as my case was totally the reverse. I was in a state of excitement resembling the effect of some spirituous liquor. I suffered no pain; I experienced only a strong pressure round my head, as if an iron circle had been bound about it. I232 spoke with the workmen, and had some difficulty in hearing them. This difficulty of hearing rose to such a height, that during three or four minutes I could not hear them speak. I could not, indeed, hear myself speak, though I spoke as loudly as possible; nor did even the great noise caused by the violence of the current against the sides of the bell reach my ears.”

The effect thus described by Dr. Colladon is different from that anticipated by Dr. Wollaston. He was not merely deaf to low tones, but to all sounds whatever; and I have found, by repeated experiment, that my own ears become perfectly insensible even to the shrill tones of the female voice, and of the voice of a child, when the drum of the ear is thrown into a state of tension by yawning.

With regard to sounds of high pitch at the other extremity of the scale, Dr. Wollaston has met with persons, whose hearing was in other respects perfect, who never heard the chirping of the Gryllus campestris, which commonly occurs in hedges during a summer’s evening, or that of the house-cricket, or the squeak of the bat, or the chirping of the common house-sparrow. The note of the bat is a full octave higher than that of the sparrow; and Dr. Wollaston believes that the note of some insects may reach one octave more, as there are sounds decidedly higher than that of a small pipe, one-fourth of an inch in length, which he conceives cannot be far from six octaves above the middle E of the pianoforte. “The suddenness of the transition,” says Dr. Wollaston, “from perfect hearing to total want of perception, occasions a degree of surprise,233 which renders an experiment on this subject with a series of small pipes among several persons rather amusing. It is curious to observe the change of feeling manifested by various individuals of the party, in succession, as the sounds approach and pass the limits of their hearing. Those who enjoy a temporary triumph are often compelled in their turn to acknowledge to how short a distance their little superiority extends.” In concluding his interesting paper on this subject, Dr. Wollaston conjectures that animals, like the grylli (whose powers of hearing appear to commence nearly where ours terminate), may have the power of hearing still sharper sounds which at present we do not know to exist, and that there may be other insects having nothing in common with us, but who are endowed with a power of exciting, and a sense of perceiving, vibrations which make no impression upon our organs, while their organs are equally insensible to the slower vibrations to which we are accustomed.

With the view of studying the class of sounds inaudible to certain ears, we would recommend it to the young naturalist to examine the sounds emitted by the insect tribe, both in relation to their effect upon the human ear, and to the mechanism by which they are produced. The Cicadæ or locusts in North America appear, from the observations of Dr. Hildreth,28 to be furnished with a bagpipe on which they play a variety of notes. “When any one passes,” says he, “they make a great noise and screaming with their 234air bladder or bagpipes. These bags are placed under, and rather behind, the wings in the axilla, something in the manner of using the bagpipes with the bags under the arms—I could compare them to nothing else; and, indeed, I suspect the first inventor of the instrument borrowed his ideas from some insect of this kind. They play a variety of notes and sounds, one of which nearly imitates the scream of the tree toad.”

Among the acoustic wonders of the natural world may be ranked the vocal powers of the statue of Memnon, the son of Aurora, which modern discoveries have withdrawn from among the fables of ancient Egypt. The history of this remarkable statue is involved in much obscurity. Although Strabo affirms that it was overturned by an earthquake, yet as Egypt exhibits no traces of such a convulsion, it has been generally believed that the statue was mutilated by Cambyses. Ph. Casselius, in his dissertation on vocal or speaking stones, quotes the remark of the scholiast in Juvenal, “that, when mutilated by Cambyses, the statue, which saluted both the sun and the king, afterwards saluted only the sun.” Philostratus, in his life of Apollo, informs us, that the statue looked to the east, and that it spoke as soon as the rays of the rising sun fell upon its mouth. Pausanias, who saw the statue in its dismantled state, says, that it is a statue of the sun, that the Egyptians call it Phamenophis, and not Memnon, and that it emits sounds every morning at sunrise, which can be compared only to that of the breaking of the string of the lyre. Strabo speaks only of a single sound which he heard; but Juvenal, who had probably heard it often235 during his stay in Egypt, describes it as if it emitted several sounds: