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 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 day 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 some times 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 par

ticular 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 independent 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 one 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 may 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.

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."

As Sir David has not advanced all his proofs and experiments in confirmation of his theory, that the "mind's eye" is really the "body's eye," we can neither be satisfied of its truth, nor fairly attempt to disprove its accuracy. Yet we venture to make one remark. Memory is the preservation of the impressions which have formerly been transmitted to the mind by the senses from without, or from the organic functions and sympathies within; such, at least, would be our definition of it. Memory is, therefore, a composite thing, the re

sult of different impressions. Our remembrance of a friend implies our recollection of his external form, his voice, his character, and so on; our recollection of a place, our native village for example, is the recollection of the pleasant meadows which we saw, the bubbling rivulet which we heard, the hawthorn and the wild rose which we smelt, the balmy freshness of the air, so grateful to our sense of feeling, and the thousand things which appeal more particularly to the heart, and tell it of other and happier days.

Let us apply these facts to the explanation of spectral illusions. If a person sees before him the image of an absent or departed friend, we suppose that it is a capricious and fitful exercise of the memory. If the person does no more than see the spectre, it is manifest that this exercise of memory is but partial, a recal of only the impression formerly received through the sense of vision. Now Sir David believes that this is absolutely seated in the organic retina. But suppose the spectre a more comprehensive one, that we not merely see its form, but appear to hear its voice and listen to its admonition-or in place of imagining, as Sir David has beautifully done, the panorama of Mont Blanc gliding in, for a moment, between us and St. Paul's, let us venture to mount to the summit of that cathedral-let us glance at the mighty city at our feet, and, whilst musing on its future destiny, transplant ourselves in fancy to Rome-see the city of Romulus spread before us, its triumphs, its glories, and its crimes-pass in rapid review its rise, its zenith, and its fallgo to battle with Romulus, to the groves with Numa-hear Cicero in the Senate, and denounce his proscription-feel as true Romans felt, when tyrant after tyrant assumed the purple to commit crimes on an imperial scale-shudder at the march of Attila, and grieve at the sacking of the Bourbon-let us do all this, and we shall but have indulged in a reverie, in which many with classical recollections have indulged. Now the simple recollection of a striking landscape differs from this historical effort of memory and fancy only in ex

tent-and the same may be said of the mere recal of the form of an absent person, or of this with the addition of other attributes, requiring the presence of other senses besides vision for their perception.

Now shall it be said, in this wider view of the subject, that the perception of what we do ideally perceive is not in the brain, but that the organic sense is the immediate and efficient agent? In recalling the image of the country, is the perception of the sounds which we seem to hear, of the fragrance which we seem to smell, seated in the organs of hearing and smelling? We apprehend not. It must be borne in mind, that the sense is not actually existent in the organic apparatus by which it is obtained. The eye is the organ of vision, but the actual faculty is, in all probability, seated in the brain. The eye is the apparatus by which visual phenomena make their impression on the brain; that impression having been made, the memory of it remains, and surely in the brain. The objection urged by Sir David Brewster, that spectra are affected by the motions of the eyes, in fact, by what modifies actual vision, is specious, but not conclusive. It must be recollected that impressions received through the medium of the eye, are impressions which have been produced and regulated by the laws of light. If the memory of them be accurate, it is the memory of impressions which, having been obedient to those laws, must continue to be so; if not, the memory is false, the spectre conjured up is monstrous, and, as it would sin against our experience, its very irregularity would convict it. Again, the spectre of a visual object, if raised by the mind, must be presented to the eye; if not, it could not possibly be the representation of what we had formerly seen, nor would it evince the agency of memory. We throw out these hints, without pretending to have paid great attention to the subject; and as neither ourselves nor our readers are particularly partial to metaphysical reasonings, in the pages of a practical journal, we pass to something else.

There is an optical illusion which

may readily occur, which has attracted some attention, and of which it is not amiss for medical men to be aware; it is that by which depressions are conceived 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 a 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 taking in

the whole. But why these things should so happen, he did not pretend to determine.'

The best method of observing this deception, is to view the engraved seal of a watch with the eye-piece of an achromatic telescope, or with a compound microscope, or any combination of lenses, which inverts the objects that are viewed through it. 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 L R, 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 R L, and as every body with its shaded side farthest 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."+

II. ACOUSTIC ILLUSIONS.

The doctrines of sound have been long and successfully applied to the purposes of delusion, and its phenomena have contributed in a very great degree to establish the influence of superstitious feelings, and to enable the weak to deceive themselves and others. Within a certain angle sounds are imperfectly discriminated, and we judge with much uncertainty of their direction and distance; an uncertainty which 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 calin day each of them speaks in succession, we shall not be able with closed eyes to determine from which of the ten men any of the sounds proceeds, 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 will 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 would 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 speak 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.

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 directions of the sound is much greater than the angle of uncertainty. Mr. Dugald Stew art 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, rivetted the eyes of his audience on the instrument, though every sound they heard proceeded from his own mouth. The late Saville 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."

The actual condition of the vocal organs in the exercise of ventriloquism is certainly ill understood. Let us hear what Sir David has to say.

that his art

quists possess the power of fetching a voice from within; he caunot 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 phenomena of ventriloquism which he has heard. This opinion, however, is strongly opposed by the remark made to Mr. Stewart himself by a ventriloquist, 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.* 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."

There is an interesting chapter on the various mechanical attempts which have been made to imitate the voices of animals and of man. M. Vaucanson's flute-player and tabor-player were extremely ingenious automata, but a talking one, if perfected, would be infinitely more astonishing. Sir David, indeed, has no doubt that "before another century a talking and a singing machine will be numbered among the conquests of science." M. Recupelen actually constructed an automaton which could pronounce words, and even sentences, as opera, astronomy, venez avec moi à Paris, &c. It is curious to observe the effects of great elevations and the re

Edinburgh Journal of Science,