PROCEEDINGS OF THE SOCIETY. MEETING OF 9TH APRIL, 1884, AT KING'S COLLEGE, STRAND, W.C., THE PRESIDENT (THE REV. W. H. DALLINGER, F.R.S.) IN THE CHAIR. The Minutes of the meeting of 12th March last were read and confirmed, and were signed by the President. The List of Donations (exclusive of exchanges and reprints) received since the last meeting was submitted, and the thanks of the Society given to the donors. Hinde, G. J.-Catalogue of the Fossil Sponges in the Geological Department of the British Museum (Natural History). 248 pp. and 38 pls. 4to, London, 1883 Microscope by Chevalier.. Martin, B.-System of Optics. xxiv. and 295 pp., xxxiv. pls. Collection of Australian Reptiles and Amphibia From The Trustees. Mr. W. Forgan. Ditto. Mr. W. E. Pickels. The President said that since their last meeting they had received an intimation from the R. Accademia dei Lincei of Rome, of the death of Signor Quintino Sella, who as President of the Academy was one of their ex-officio Fellows. He proposed that a vote of condolence should be forwarded to the Academy expressing the sympathy of the Society with the Academy in the loss of their illustrious President. Dr. Anthony having seconded the proposal, it was carried unanimously. The President proposed that as they were favoured by the presence of Dr. Carpenter, who intended to deal with the subject of binocular vision in the Microscope, the other business on the agenda should be postponed. This was approved by acclamation. Dr. Carpenter then addressed the meeting "On the Physiology of Binocular Vision with the Microscope," illustrating the subject by some large photographs, drawings on the black-board, &c. He said: The reason of my venturing to offer to the Society the views which I entertain upon the subject specified as the title of this communication, is that in the last number of the Journal' of the Royal Microscopical Society, at the end of a paper by Prof. Abbe, a doctrine is put forward on the nature of Stereoscopic vision with the Microscope, which appears to me to be inconsistent with our knowledge of the physiology, and also with our experimental knowledge of the pheno mena, of stereoscopic vision. It is not, I think, so much a question of optics, as of the physiology of vision. If it was one of optics, I should certainly not venture to put myself in antagonism with one who is probably the greatest living master of the theory of the Microscope. But I think I shall be able to show that it is essentially a question of physiology, and in part also of psychology. Ever since Wheatstone's invention of the Stereoscope, something like fifty years ago, I have had the subject constantly before me: and from the first introduction of the binocular Microscope, I have used it continually for objects of suitable character. So completely, indeed, am I accustomed to it, that when I look at some of the same objects under the monocular Microscope, I scarcely know them again. The manner in which we form our visual conceptions from impressions produced upon the retina, is a matter of both physiology and psychology, lying on the border line between the two. Our visual conceptions are formed by the process which is known as "suggestion"; that is, they do not necessarily conform to the visual impressions produced upon the retina, but they are suggested to us by these visual impressions; and it sometimes occurs that our conceptions are erroneous. All who have given attention to the physiology of vision, agree in considering our ordinary interpretations of the solidity of an object placed before us, to be dependent upon a mental co-ordination of our visual and tactile sensations. A child moves its hands towards an object presented to its vision, and educates itself to a conception of its form by the conjoint use of its sight and its touch. It has happened that in some cases persons have obtained sight for the first time, having been born blind, at an age when they have been able to record their impressions of objects presented to their sight, and to manifest their difficulties of interpretation. Many years ago I had the opportunity of observing a child three years old, who had been operated on for congenital cataract. He was too young to describe his impressions to us, but we could observe when he was guided by sight and when by touch, and it was very interesting to watch him under these circumstances. In the lodging where he was staying whilst under treatment, everything about him was strange, and he used his sight and his touch conjointly in familiarizing himself with them until he had learned to correlate the two impressions. But when taken to his own home where the surroundings were perfectly familiar to him, he was for some time entirely guided by touch; he seemed to be quite puzzled by the sight of them, and often shut his eyes in order to understand where he was. Many of you have heard of the case recorded by the celebrated Cheselden, the subject of which, being much older, could describe his own sensations. For a long time after he could see distinctly, he could not distinguish solid objects by vision alone from flat pictures. Not very many years ago, the case was published of a young woman who from birth had possessed enough sight to enable her to distinguish light from darkness, but who could not see the form of any object about her. She had been accustomed to work with her needle; and her thread, needle, scissors, balls of cotton, &c., were all perfectly well known to her by touch. You would suppose that the peculiar form of a pair of scissors, as suggested to the mind through the medium of touch, would be recognized through the sight more readily than anything else; and yet when it was first shown to her, she utterly failed to recognize it as the implement which she had been in the habit of handling. This recognition of a solid form from a visual picture, then, is the result of the experience we gain in very early life, from the association of the mental impressions made by the retinal pictures with those we obtain through the sense of touch,-by which I mean not only the contact with the fingers, but the muscular action which gives movement to them, so that, in course of time, the visual picture comes to suggest the solid form of the object to the mind. Our best evidence of this is derived from pictures obtained by means of photography; especially those in which the relations of light and shade are strongly brought out; for these pictures suggest the idea of solidity much more perfectly than any others can do. Some of you will probably remember the old Dioramic pictures in the Regent's Park, with their wonderful appearance of solidity, especially in the case of architectural designs; the impression produced being so entirely that of solidity, that it was only by moving the head from side to side that the illusion was detected. These pictures were based on photographs; Daguerre and others having worked out the original "daguerreotype" process for the purpose of producing them most effectively. In ordinary drawing and painting, an artist is subject to continual changes in the conditions of the light and shade, even in the course of half an hour; and therefore no painting, except one by artificial light, can give a true representation of light and shade at any particular moment. Therefore it is that photographs of many subjects are most wonderfully illusive, and most especially so when they are looked at with only one eye. The explanation of this effect is, that when you look at the picture with both eyes, and it is tolerably near to you, you are forced to see it as a flat surface; but when you shut one eye and keep the head still, you lose the power of measuring relative distances; and a visual conception of solid form is suggested by its chiaroscuro and its perspective. If you look, for example, with one eye at the photographs of relievos hanging upon the opposite wall, you will, if you have not tried the experiment before, be astonished' at the way in which the figures seem to stand out with all the effect of stereoscopic relief. This is a pure case of mental suggestion; and is due to the perfect similarity of the photograph to the retinal picture produced by natural vision of the object itself upon a single eye. The camera, like the eye, projects a flat picture, which is recorded by photography; and you have then permanently just the picture which one eye would form of the object. You look at this with one eye, and, trained by experience, you interpret what you see according to your preconceived conceptions. A similar effect is obtained when you look at such pictures with both eyes, at a distance great enough for the axes of the eyes to be virtually parallel. I remember some large imitation relievos on the cornices of some of the apartments in the Louvre at Paris, and some still larger pic tures of the same kind in the Bourse, by which the impression of solidity is so well given, that, though the paintings are quite flat, they are generally taken by strangers for real relievos. I have a photograph of a figure in such low relief, that, looking at it with both eyes at a distance of only two feet, you could almost swear to its solidity; the suggestion of solidity given by its lights and shadows being so vivid, as to overcome the corrective effect of the binocular perception of its flatness. I dwell upon this point, because it underlies the whole inquiry before us. I have here four large photographs of plaques representing the Four Seasons, with the ornamentation and figures in high relief. When you look at three of these with one eye, you will scarcely be able to persuade yourselves that you are not seeing actual relievos, so vividly do the figures stand out. But I have hung one of them upside down; and though you may not all see it as I do, I think the impression upon most persons will be that the figures are hollowed out, instead of raised. In each case the illusion depends mainly upon the light; and it is most complete when there is but one source of light in the room, corresponding with the lights in the photograph. The mental impression is entirely due to suggestion; you know the position of the light, and can tell in which direction the shadows would fall; and when the shadow is made to fall as it would if the object were hollow, then the mind interprets the object as such. Another remarkable instance of suggestion is afforded by this figure of a rhomb (fig. 80), which, as you look at it, may seem to change from one position to another, sometimes appearing to stand upon its narrow side, at other times to be lying on its broad side. Sir D. Brewster FIG. 80. says that the perception changes from one to the other, as you feel your mind changing; but I believe that the perceptional and therefore the mental change is the result of the wandering of the eye from the point a to the point b; for I have never failed to see one or the other aspect, by making my eyes converge upon one or the other of these two points, which then becomes the salient angle. This is a case in which two different effects of projection may be produced by the same visual impression; a consideration much dwelt upon by Sir Charles Wheatstone in his original memoir,* as proving that the conception of solid form is visually suggested to the mind, not a mere optical effect. I now come to the subject of Binocular Stereoscopic vision, which was first elucidated in that memoir. Painters had long been aware of the fact, that if you look at a near object with both eyes, you form different pictures with your two eyes. How is it, then, that we are not * "On some remarkable and hitherto unobserved phenomena of Binocular Vision." Phil. Trans., 1838, pp. 371-94. puzzled by these different pictures, presented to the mind at the same time? Wheatstone applied himself to the study of this question; and in the course of his investigations it occurred to him that the dissimilarity of the two pictures was really the cause of the sense of projection; and that though we have this sense with a single eye, it is in such case by no means so unmistakable. He therefore reasoned in this way; if you draw two pictures of an object, one as it appears to the right eye, and the other as it appears to the left, and then throw the images of these dissimilar pictures upon the two eyes respectively, you will get a solid effect. The original form of the Stereoscope was a reflecting apparatus, consisting of two mirrors placed together at a right angle, so that each reflected the image of its own picture direct to its own eye; and with this instrument Wheatstone found that two mere outlines of a solid, drawn as already described, and reflected so that each was seen only by the eye for which it was drawn, resulted in the production of a perfect perception of the solid form. No one welcomed this discovery more than Sir David Brewster; who said that it was the greatest that had been made in vision since the time of Newton. This combination of two dissimilar pictures is the fundamental principle of the Stereoscope. In the form of that instrument now familiar to you all, a pair of small photographic pictures, taken in different perspectives, are brought one before the right eye, the other before the left, by two halves of a double-convex lens placed back to back, so as to act both as prisms and as magnifiers. The points of view from which the two pictures are taken, are generally, I believe, about 15° apart; that being the usual angle of convergence of the axes of the eyes at the ordinary reading distance. The late Mr. Claudet, who paid a great deal of attention to this subject in relation to portraiture, tried various angles; and having taken pictures at 5°, at 10°, at 12°, at 15°, and at 20°, he found that 5° gave very little projection, 10° was more satisfactory, but 12° was much better; and for people with nearly approximated eyes it was found to be sufficient; but for most people, 15° was required to bring out the full stereoscopic effect, whilst if he widened the angle to 20° all the projecting parts came out with ludicrous exaggeration. (I have an early stereoscopic photograph of an equestrian statue of Napoleon, showing this exaggeration in a very marked degree, the two pictures having been taken at too wide an angle.) As an illustration, take a truncated pyramid which is placed end-on before one eye, as is shown in fig. 81 a; with that eye alone you would be unable to measure the relative distances of its parts, and the borders of its base and truncated top would appear like two squares symmetrically placed one within the other. But if placed in front of the nose, the right eye would see more of the right side of the pyramid (as in the fig. 81 c), whilst the left eye will at the same time see more of the left side of it (as in fig. 81b); and if these two pictures are put into the Stereoscope, and each is seen at the same time -the one by the right eye, and the other by the left-the apparent solidity of the figure is brought out perfectly; that is, these two dissimilar pictures, viewed simultaneously, suggest to the mind a con |