We must now pass on to the concluding part of the subject—viz., " single vision with two eyes.," in treating of which we shall have recourse, almost exclusively, to the masterly researches of sir Charles Wheatstone, of whose admiiable discoveries in this department of knowledge we have already had occasion to speak in the article STEREO SCOPE (q.v.). It will be obvious to those who have read what is there stated, that the question of single vision with two eyes is naturally divisible into two classes—the first including those cases in which the optic axes are parallel, and the retinal images exactly alike; and the second, those in which the optic axes are convergent. and the retinal images dissimilar. Now, to see an object double is to see it in two different places at the same time; and therefore, if it can he shown that by the law of visible direction an impression upon corresponding points of the two retinae is necessarily referred to the same place, this will account for our single vision of the object at that spot. And on consideration, it will be plain that this is really what happens When the optic axes are parallel, and the images indentical. But it is also evident that this explanation does not apply to the second class of instances; in which the only visible point which depicts itself on corresponding portions of the two retinue, is that point to which the optic axes are directed. All other points, whether situated before, beyond, or in the plane of the horopter,* are projected upon non-corresponding points of the retinae; and as these con ditions were presumed to be inconsistent with single vision, it was asserted by Aquilon his that objects arc seen single only in the plane of the horopter (it has since, with greater consistency, been said, only at the point of intersection of the optic axes); but that this is not true is evinced by our common experience that, without movement of the optic axes, we enjoy a certain limited field of distinct vision. Its complete refuta tion, however, is involved in the theory of stereoscopic vision, which may be thus explained: Let the optic axis of the right eye (R) and of the left eye (L) be converged on the point A; suppose another point B, slightly to the left, and in advance of A; and then through the point B draw lines from L and R respectively intersecting the plane of the horopter in r and /(fig.1). Now, if two diagrams, SS, be prepared (the one representing land A, and the other r and A), and these be presented to their appro priate eyes in the stereoscope, with the view of each eye limited to its own picture, the points r and I will be seen as a single point, situated not on the paper, but in advance of it, in the point of inter section of the lines of visible direction, indicated in the above construction by Er and Ll. If the point B be supposed beyond A, and also to the left of it the line's drawn from L and R to B will intersect the plane of the horopter in 1 and r (fig. 2); and stereoscopic pictures SS pre pared tinder these converse conditions will exhibit the points 1 and r as a single point placed behind A at the point of intersection of the lines drawn from L and R respect ively. This simple rule involves, as it seems to us, the true principle of the stereoscope; and it is capable of being applied to the most complicated stereoscopic pictures. Feu', in a stereogram, let 1 and r stand for identical parts of the left and right pictures respect ively, and suppose the pictures superposed; those parts which, read off laterally from left to right, stand in the order Zr will recede, and those in the order 2./ will protrude (relatively to those parts of the pictures in which r and / absolutely overlie each other),, when the pictures are viewed together in the stereoscope. It appears, then, that vision of the third dimension of space is directly obtained by impressions on non-corresponding retinal points; the proof of this being given in the appearance of solidity experienced in the stereoscope, although perfectly plane representations are alone employed; but it would be an error to suppose that this non-correspondence is without a limit; and the question still remains, to what extent the retinal points affected may be non-correspon dent, consistently with single binocular vision. Without attempting to propose any definite solution of this difficult question, it may, we think, be considered as highly probable that this limit is determined by the same law which regulates our distinct vision of objects by means of rays inexactly focussed on the retina;; for, according to Mr. Abbott, " as long as the rays are contained within the area of one sensitive mini mum, the sensation will be that corresponding to the vision of a point;" and " a certain amount of dispersion does not interfere with distinct vision." It seems certain that the double perception which is experienced of the further of two objects, when the optic axes are fixed on the nearer, or vice versa, can only arise when the object, thus doubled, is situated within the angle of the optic axes (whether before or beyond their intersec tion); for under these circumstances only, the sensitive points affected are not simply non-correspondent, but are utterly diverse, being in fact on different sides of the centers of the retina in the two eyes. That the law of projection of the various points com posing the relief of a stereoscopic object is correctly stated above, is strongly corrobo rated by a curious experiment of sir C. Wheatstone 's in which solid objects are placed iu the stereoscope, instead of pictures. As, for example, two skeleton cubes, so placed, that when the optic axes converge upon them, identical pictures are depicted on the retina; in which case, all appearance of relief vanishes, and a perfectly plane perspective repre sentation of a single cube is alone visible; the reason being, that the lines of visible direc tion for each point intersect each other, neither before or byond, but in the plane of the horopter, where, accordingly, the object is seen as a perspective projection. The same rule holds when the right and left eye pictures are interchanged, for the pictures being sup posed, as before, to overlie one another, the parts Zr become now r1; that is, instead of having their point of intersection beyond the plane of the horopter, they have it before ,that plane; and this, mutatis mutandis, being true of all the parts of the pictures, the stereoscopic resultant is the converse of. that which would be perceived but for this
abnormal arrangement. In these phenomena, named by sir C. Wheatstone the "con version of relief," and copiously treated of by him in his various papers, the usual rela tions of distance also are reversed, the nearer parts being seen as further, while the latter are perceived to be of larger dimensions than the former; and,. the same principle being applied to the vision of solid objects by means of an instrument called the pseudoicope (q.v.), also invented by sir C. Wheatstone, they are seen as if turned inside out, and under divers other aspects of a most extraordinary character, some account of which will be found in the article just cited. But, as to many of them, it is proper to mention, that the facility of conversion is found to depend, not on the optical conditions, which, of course, remain invariable, but upon mental conditions, as, for instance, previous famil .
iarity or otherwise with the converse forms suggested; in short, upon our previous thuta experience.
We have not yet considered those cases in which the retina] pictures are identical, arid the optic axes convergent. In these, the law is, that the object is seen in the plane of the horopter, as is conclusively proved by a beautiful experiment, suggested by sir D. Brewster. If, while looking at a wall-paper, consisting of a small pattern, contin Bally repeated at intervals not exceeding 2i in. from center to center, we cause the eyes to converge to a point in front of the wall, the paper will appear to advance to that point, and will there be plainly visible, in spite of the contradiction of the touch, which, of course, cannot feel the wall where it is seen; while, on the other hand, the eye can perceive no wall in the place where the touch affirms.ff to exist. Tlic converse of this experiment, although more difficult to perform, is equally curious and instructive. It has also been shown by sir C. Wheatstone, that if an increasing convergence of the eyes be unaccompanied by its usual concomitant, a corresponding enlargement of the retinal pictures, the object is seen as if continuously diminished in all its proportions, albeit the size of the retinal images remains unaltered. This experiment, which, with sev eral others of almost equal interest and importance, may be performed by means of the stereoscope, also establishes that every degree of convergence of the optic axes is asso ciated with the particular adaptation of the eye suited for distinct vision at that distance. This adaptation is, of course, directly dependent upon the divergency, less or greater, of the impinging rays, and this again stands in a necessary relation to the dis tance, real or virtual, of the point from which they diverge; a branch of the subject to which we have already given sufficient prominence. All observations and experiments concur in showing that a part of the highest importance is played in vision, by the con vergence of the optic axes, in particular, in so far as this is conjoined with a difference between the two retinal pictures; and, for this reason, it matters but little that we can not, within our present limits, enter on a discussion of the evidence obtained from those persons, blind from birth, who have gained their sight by means of a surgical operation; for, in almost every case, only one eye at a time was operated upon, and the information then obtained from the patients, under circumstances of so much difficulty, is admitted on all hands to be of a very dubious and unsatisfactory character.
By mere modification of the light incident upon the eye, the same visible objects may be seen under infinite variations of figure, situation, and magnitude; while, at the same time, their real figure, real situation, and real magnitude, as apprehended by the touch, shall remain unaltered; but these phenomena, artificially induced, argue nothing against the general fact, that under normal circumstances we find. in the very place of the visible objects, those "dynamical qualities" which form the sum-total of our tactual experiences. To Berkeley is due the credit of having first pointed out the origi nal entire disconnection and subsequent intimate blending of the two sets of experience —visive and tactual; but, if the views here proposed be correct, he erred in supposing that our realization of the geometrical proportions and relations of visible objects, is dependent on the suggestions of touch, and not upon the exercise of a primitive and inherent function of sight. To the popular view, the objects of sight have a positive and equal existence in absolute darkness, and are simply rendered visible by the light; whereas, they are, in truth, the light itself variously modified. But, in conclusion, while fully admitting that light and its modifications, viz., color in all its varieties, form the sole objects of sight, we venture to maintain that we only know color by our percep tion of it, as making up, by its superficial distribution, the visible form and shape of the objects of the outer world; and that this our perception of the shape, relative magnitude, and situation of visible objects is immediate, and strictly regulated by the laws of light in relation to the visual organ, irrespective of, and even in opposition to, tactual expe rience; but, at the same time, we bold that to the touch alone, we owe our ever-present and ineradicable belief,. that these visible appearances have underlying them a material ity which we cannot conceive as actually modified concurrently with those changes of form and magnitude, which are perpetually occurring in relation to our faculty of sight; and therefore, in all questions which relate to real size or distance, we necessarily have recourse in thought to those qualities of matter which are apprehensible by the touch.
That an instinctive power of direct visual perception is possessed by the young of the lower species, is not denied by any; whether a like power has been bestowed upon man, we must now leave to the consideration of the philosophic reader.
See Berkeley's Theory of Vision; Wheatstone On the Physiology of Vision; review of Berkeley's Theory of Vision, by Samuel Bailey; review of the last-named work by J. S. Mill, in his dissertations and discussions; Sight and Touch, by T. K. Abbott; Helmholtz's Popular Lectures; art. on vision by Sully, in "Mind," Nos. IX. and X.