For the production of single vision, it is necessary that the muscles which move both eyes should act in perfect concert. The effect of this is, that the axes of the eyes converge towards the object to which they are adjusted, and the image falls on corre sponding parts of the two retinae. Let A B be the two eyes, and c any object before them. Then A c, B C are their axes, which meet in c. An image is consequently pro duced in each eye which will correspond with the perspective projection of the object from the points A and a. If the two images are un symmetrically placed, so that the axes meet either before or behind the object, a double sensation is excited, as happens in strabismus, and the double vision of intoxication. The upper sides of both retina? correspond, as do the lower ; and the outer side of each corresponds with the inner side of the other ; but the objects and corresponding points of the retina should lie in a certain circle, designated the horopler ; a circle which passes at once through the point of coincidence, 1, of the visual axes, I a, b, and the points of decussation, c a', of these axes with the lines of direction.
are more dissimilar as the convergence of the optic axes b:come sgreater. Fig. 888. repre sents the two perspective projections of a cube. a is that seen by the right eye, and A that presented to the left eye, the figure being supposed to be placed about seven inches im mediately before the spectator and viewed with each eye alternately, the other being closed, and the head kept perfectly steady.
Let c c be the centres of the eyes A 13, 1 the point regarded, 1 c c' the horopter, a and b the points of the retina on which the axes of the eyes terminate; and let nt be a second point in the horopter. The point 1 appears upon the axial points a and b, the point nt at o and p ; a line betwixt /az will form the chord of the arc of the horopter lying between 1 and nz ; and as all triangles drawn upon the arc of a circle have equal peripheral angles, so is the angle 1 c zn equal to the angle / Both are farther equal to the opposite angles o c a, p c' b. Moreover o c is equal to p c, and a c to b c', as radii proceeding from the centres c and c' of the retinal circles, which in each eye have an equal circumference. Consequently o is just as far from the axial point a, as p is from the axial point b ; and so identical or corresponding points of the retina are affected by the rays proceeding from both 1 and za. It is, however, to be borne in mind, that though impressions made on non-cor responding points of the middle portions of the two retina; are perceived as two, impres sions made on non-corresponding points of the circumferential parts are not so perceived, the distance between such points being within certain limits.
When an object is viewed at so great a distance that the optic axes of both eyes are sensibly parallel when directed towards it, the perspective projections of it seen by each eye separately are similar, and the appearance to the two eyes is precisely the same as when the object is seen by one eye only. But this similarity no longer exists when the object is placed so near the eyes that to view it the optic axes must converge : under these con ditions a different perspective projection of it is seen by each eye ; and these perspectives Mr. Wheatstone has shown that the single
sensation excited by these two images is that of a third image different from them both, but excitable only by both of them at once, and attended with the notion of solidity or projec tion in relief. This he has illustrated by a most ingenious instrument called the stereo scope. Accurate representations are drawn of the appearance presented by all object of three dimensions, when viewed by each eye at a short distance. These drawings are then placed symmetrically in the right and left compartments of a small box, so as to be reflected by sloping mirrors to the eyes of the observer, who must place them as near as possible to these mirrors ; then, by moving sliding panels to or from him, he will find a position, and one only, in which the bin ocular image will be seen single, of its proper magnitude, and without fatigue to the eye, because in this position only the ordinary re lation between the magnitude of the pictures on the retina, the inclination of the optic axes, and the adaptation of the eye to distinct vision at different distances, are preserved. It being thus shown that there is an essential difference in the appearance of objects when seen with both eyes, and when only one eye is employed; and that the most vivid belief of the solidity of an object of three dimensions arises from two different perspective projec tions being simultaneously presented to the mind, the question arises, how is it that per; sons who see with only one eye form correct notions of solid objects, and never mistake them for pictures ? and how happens it that a person having the perfect use of both eyes perceives no difference in objects around him when he shuts one of them ? To explain these apparent difficulties, says Mr. Wheat stone, it must be kept in mind that, although the simultaneous vision of two dissimilar pic tures suggests the relief of objects in the most vivid Intinner, yet there are other signs which suggest the same ideas to the mind, and are less liable to lead the judgment astray in pro portion to the extent of our previous expe rience. The vividness of relief arising from the projection of two dissimilar pictures, one on each retina, becomes less and less as the object is seen at a greater distance from the eyes, and entirely ceases when it is so distant that the optic axes are parallel while regarding it. We see, with both eyes, all objects beyond this distance precisely as we see near objects with a single eye ; for the pictures on the two retin? are then exactly similar, and the mind appreciates no difference, whether two iden tical pictures fall on corresponding parts of the two retina?, or whether one eye is im pressed with only one of these pictures. A person deprived of the sight of one eye sees therefore all external objects, near and remote, as a person with both eyes sees remote objects only : but that vivid effect, arising from the binocular vision of near objects, is not per ceived by the former, who, to supply the deficiency, resorts to other means of acquiring more accurate information ; and of these the motion of the head is the principal.