ART. V.-1. Contributions to the Physiology of Vision. Part I. On some remarkable and hitherto unobserved Phenomena of Binocular Vision. By CHARLES WHEATSTONE, F.R.S. Philos. Transact. 1838.

2. Contributions to the Physiology of Vision. Part II. By CHARLES WHEATSTONE, F.R.S. Philos. Transact. 1852. 3. The Stereoscope; its History, Theory, and Construction, with its Application to the Fine and Useful Arts, and to Education. By Sir DAVID BREWSTER, K.H., D.C.L., F.R.S., M.R.I.A. London: 1856.

4. Observations on Binocular Vision. By Prof. WILLIAM B. ROGERS. From the American Journal of Science and Arts, 1855.

5. Essai sur les Phosphènes, ou Anneaux Lumineux de la Rétine, considérés dans leurs rapports avec la Physiologie et la Pathologie de la Vision. Par le Dr. SERRE, d'Uzès. Paris: 1853.

6. Recherches sur la Vision Binoculaire, Simple et Double, et sur les Conditions Physiologiques du Relief. Par le Dr. SERRE, d'Uzès. Bruxelles: 1856.

7. On the Phenomena and Mechanism of the Focal Adjustment of the Eye to distinct Vision at different Distances. By Prof. ALLEN THOMSON, M.D., F.R.SS.L. & E. Glasgow:

1857.

8. On the Phenomena of Relief in the Image formed on the Ground Glass of the Camera Obscura. By A. CLAUDET, F.R.S. From the Proceedings of the Royal Society: June 18. 1857.

9. On the Stereomonoscope: a new Instrument by which an apparently Single Picture produces the Stereoscopic Illusion. By A. CLAUDET, F.R.S. From the Proceedings of the Royal Society: April 15. 1858.

WHY, having two eyes, we ordinarily see the objects around

us not double but single, is a question which most persons of ordinary intelligence have asked themselves at one time or another. Why, the picture on the retina being inverted, our vision is nevertheless erect, is an inquiry that naturally occurs to every one who is acquainted with the optical fact of the inversion. But why, the two retinal pictures of any solid body within a moderate distance of the eyes, being sensibly different, we are not only unembarrassed by that difference, but gain a

much more complete and certain conception of the shape of that body than we can acquire by the use of either eye singly, -is a consideration which, obvious as is its importance when once suggested, seems to have presented itself to comparatively few persons, and never to have been seriously examined until it was taken up by Mr. Wheatstone about twenty-five years ago. These three problems are more closely linked together than at first view they seem to be. They all belong to that borderregion between the corporeal and the mental, the physiological and the psychical, which has ever been most fertile in controversy; one party contending for the sufficiency of physical explanations, whilst the other affirms that their solution lies altogether within the domain of metaphysics, neither optics nor physiology having anything to do with it. Thus it has come to pass that one set of philosophers has reasoned upon the phenomena of vision, as if the retinal picture formed by the optical instrumentality of the eye were daguerreotyped (so to speak) on the mind, just as the photographer transfers the picture obtained by his camera from one sensitive surface' to another: whilst another set has gone so far as to deny that we have any true idea either of the relative places or of the actual forms of objects, save what we acquire from the combination of tactile with visual experience, and to affirm that the infant really does see objects inverted until he has learned the truth by handling them.

The researches of Professor Wheatstone on the Physiology of Binocular Vision have added so much to our knowledge of this subject, not only by what they have themselves proved, but also by the inquiries which they have suggested to other investigators, that it has come to present itself under an aspect in many respects new; and not only can tolerably definite answers be now given to each of the questions we have just propounded, but the relative shares of the eye and the mind, of the optical instrument and the conscious interpreter of its indications, can be pretty clearly marked out. The invention of the Stereoscope, Professor Wheatstone's claim to which we regard as altogether incontestable, was not the result of accident, but the product of a train of sagacious reasoning; and whilst for that more recent modification of the instrument which has led to its wide-spread and still increasing popularity we are indebted to the ingenuity of others, its philosophic originator, leaving its practical applications to be developed by such as care to turn them to pecuniary account, has confined himself to the pursuit of the scientific inquiries out of which his invention arose. In the course of these inquiries, he has been led not merely to the

introduction of very important modifications into his original form of the stereoscope, by which the sources of our appreciation of magnitude and distance may be submitted to experimental tests; but also to the invention of an entirely new instrument, the Pseudoscope, the use of which furnishes an unequivocal demonstration that the share of the mind in the interpretation of visual impressions is far greater than those who hold that vision is to be explained on optical principles alone have been ready to admit.

The value of Professor Wheatstone's researches upon Binocular vision, however, will be best appreciated, if we first obtain a clear understanding of what Monocular vision, or vision with a single eye, can and cannot do; and the pursuit of this preliminary inquiry will give us an opportunity of noticing in their most appropriate place the highly interesting results of two sets of recent experimental investigations, the one directed to the determination of the mode in which the focal adjustment of the eye is effected, the other to the source of our appreciation of the direction of the objects which produce their pictorial impressions upon the retina.

Whilst the marvellous perfection of the eye as an optical instrument has come to be more and more appreciated with every advance in our knowledge of its action, the source of that perfection in one of its most important adjustments has until lately been an open question among physiologists. Every one who has handled a telescope well knows that if, after its focus has been so adjusted as to give a distinct image of a remote object, he directs the instrument to one much nearer, the image of the latter is indistinct until a re-adjustment has been made. by drawing out the tube so as to increase the distance between the object-glass and the eye; whilst, conversely, if, after making such re-adjustment for the near object, he turns his telescope back to the remote one, he finds the image of the latter to be indistinct, until the tube has been shortened again so as to bring the object-glass to its former distance from the eye. And it is possible, within certain limits, to deduce from the amount of the alteration required in each case, an approximative estimate of the distance of the object. Now every person who possesses ordinarily good sight has a wide range of distinct vision; being able to see objects with equal clearness (allowance being made for their difference of apparent size, and for atmospheric interference) whether they be placed at eight inches, at eight feet, at eight yards, or at eight miles from his eye. That this range can only be obtained by a special adjustment of the organ for each distance, is not merely a theoretical surmise, but may be

readily demonstrated by experiments so simple that we need scarcely describe them. And these experiments further show that the adjustment is made automatically, as the direct result of the determinate fixation of the attention on a particular object; but the change is felt to involve a certain effort, which is greater when the adjustment is made for a near than for a distant object, and which increases considerably as the object is brought closer and closer to the eye.

Various hypotheses have been offered as to the mode in which this adjustment is effected. Some have thought that the form of the eye as a whole is altered by muscular pressure, so that its axis is lengthened or shortened. Others have maintained that the distance between the crystalline lens and the retina is altered by a change of place produced in the former by a muscular apparatus within the eye. And others have attributed the result to an alteration in the curvature of the lens itself. Until recently, the second of these suppositions has been the one most generally favoured; but the last, improbable as it seems à priori, would now appear to be the true one. For Professor Helmholtz (whose researches on this point have been confirmed by Professor Allen Thomson, of Glasgow), by carefully observing the images of bright objects reflected from the anterior and posterior surfaces of the lens, when the eye had been made to adjust itself in succession for near and for distant objects, has shown that these images undergo a change in size and relative position, for which nothing but an alteration in the curvature of the lens can account. The convexity of its anterior surface is greatly augmented, the part of the iris that immediately surrounds the pupil being even pushed forwards, when the refractive power of the eye needs to be increased, so as to bring the highly diverging rays proceeding from a near object to a focus on the retina; and this convexity is proportionally diminished when the removal of the object to a distance causes the rays received from it by the eye to approximate more closely to parallelism. This automatic alteration in the curvature of the lens is one of the most marvellous pieces of self-adjustment that the human organism with all its wondrous mechanism can furnish. Let any optician set himself to devise a means of imitating it, by causing the tube of his telescope to alter its length, or the lenses to change their curvature, without any interference on the part of the observer, whenever the instrument is turned from one object to another at a different distance; and he will soon give up the solution of the problem as hopeless. What the structural arrangement is by which the alteration in the curvature of the crystalline lens is effected, still remains to be discovered; it is obviously a great

step, however, to have gained a clear idea of what to look for; since, until the nature of the change was certainly known, the physiologist who might attempt to trace out its source was truly groping in the dark.

By that combination of refractive media of different densities, and by that arrangement of the curvatures of their respective surfaces, which the optician essays to imitate in the construction of achromatic object-glasses for the telescope and microscope, as well as by the power of self-adjustment to variations of distance, it comes to pass that the eye is rendered capable of forming on the retina a picture of any object to which it is directed within the range of distinct vision, far exceeding in perfection that which the most elaborate instrument constructed by human ingenuity could present. The more carefully we scrutinise the details of this picture, the higher must our admiration rise of its marvellous exactness. As Paley says, the whole of an extended landscape shall be brought within the area of a sixpence; and yet every detail that presents a sensible magnitude shall be distinctly perceived. It is to be remarked, however, that the distinctness of the visual perception is by no means uniform over the several parts of the field of view. That portion towards which the axis of the eye is directed, is alone discerned with satisfactory clearness; all save this is but vaguely seen. Let the reader, closing one eye, fix the other upon any word in the centre of the printed page before him, and he will perhaps be surprised to find how small is the number of other words which he can read without altering the position of his visual organ,-probably no more than a word in the line above, and another in the line below, all three lying within the area of a fourpenny-piece. Still, although perfectly distinct vision is thus limited to a small space, the mind can take cognizance of the larger features of the visual picture over a much wider area; thus, keeping the eye steadily fixed upon the central word of the page, we can distinguish the lines of print over its whole surface, the form of the book, its place upon the table, the position of the table in the room and that of other pieces of furniture; and we thus gain a general idea of the nature and relations of surrounding objects, which we can complete whenever we choose by a more detailed survey. In making this survey, we direct the axis of the eye to every part of the field in succession; and thus are enabled to discern each feature with perfect distinctness, without losing cognizance of its relations to the rest.

Some physiologists have maintained that our appreciation of the relative positions of objects is derived through an intuitive