826 EYE Thus, as in fig 25, suppose two eyes looking at a single object, placed at a, b. or c. If the image of the point b fall in one eye on 6 and in the other on 7, the point 6 of the one re tina being correspondent with the point 6 of the other retina, the distance of the two images seen will be equal to the distance between 6 and 7. Again, if images of a fall on 5 and 5, it will be seen single. Further, if the image of b fall on the left eye at 6 and on the right at 4, as these two points do not corresp ond, it will appear double. And so with regard to the other retinal points indicated by th l l numbers - i FIG. 26. Diagram to illustrate the theory be studied with ThY ^id f Corresponding retinal points. (Miiller.) of fig. 26. Any object at a", or at b", or at c", will be seen simply by the two eyes A and B, as the images fall on corresponding points in the retinae, namely, aa , bb , and cc . It will be readily seen that, if the eye B were displaced, the images would not fall on corresponding points, and consequently two would be seen. The name horopter has been given to a line connecting those points in the visual field which form their image on corre sponding points of the retina. The older phy siologists first gave this name to " a straight line or plane, passing through the point of convergence of the axes of the eyes or the point to which the eyes are directed ;" but Vieth and Miiller showed that it cannot bo a straight & c line or plane, but must FIG. 27. Diagram to illustrate the simple have a circular form. horopter. Thus if the points a, b, c in fig. 26 correspond to the points a , b , c , the angles 4 and 1 in the one eye must correspond to the angles 4 and 1 in the other. Then a b being equal to a! b , the angle 1 in eye A equal to angle 1 in eye B, the angles 1 and 1 will be equal. Since the Jingles 2 and 2 are equal, the angles 3 and 3 must also be equal. In the same way, the angle 5 is equal to angle 3, For b c b c , and angle 4 = angle 4. Thus the angles 3, 3, and 5 are equal, and a b" c" can not lie in a straight line, for it is the pro perty of a circle only that angles erected on the same chord, and reach a ing the periphery have ^ no TV i- n. at the periphery equal Fl . a 28 -Diagram ilhistratmg the simple angles (Mutter s Phv llor P ter of objects at different distances siology, vol. ii. p. 1195.) from the e > es " A line joining a", b", and c" is therefore the simple horopter, and its form is illustrated by fig. 27. It is a circle, of which the chord is formed by the distance between the points of decussation of the rays of light in the eye (K A C K in fig 27). Its size is deter mined by the position of the two eyes, and the point toward which their axes converge. This is illustrated by fig. 28. Thus if a & be the distance of the eyes from each other, the circle c is the horopter for the object marked 1, the circle d fo: 2, and the circle e for 3. An object which is not found in the horopter, or, in other words, does not form an .mage on corresponding points of the retinae, is seen double. When the eyeballs are so acted upon by their muscles as to secure images on non corresponding points, and consequently double vision, the condition is termed strabismus, or squinting, of which there are several varieties treated of in works on ophthalmic surgery. It is important to observe that in the fusion of double images we must assume, not only the correctness of the theory of corresponding points of the retina, but also that there are corresponding points in the brain, at the central ends of the optic fibres. Such fusion of images may occur without consciousness, at all events it is possible to imagine that the cerebral effect (except as regards consciousness) would be the same when a single object was placed before the two eyes, in the proper position, whether the individual were conscious or not. On the other hand, as we are habitually conscious of a single image, there is a psychical tendency to fuse double images when they are not too dissimilar. (3.) Binocular Perception of Colour. This may be studied as follows. Take two No. 3 eye-pieces of a Hartnack s microscope, or two eye-pieces of the same optical value from any microscope, placee on in front of each eye, direct them to a clear window in daylight, keep them parallel, and two luminous fields will be seen, one corresponding to each eye. Then converge the two eye pieces, until the two luminous circles cross, and the central part, like a bi-convex lens, will appear clear and bright, while the outer segments will be much less intense, and may appear even of a dim grey colour. Here, evidently, the sensation is due to a fusion of impressions in the brain. With a similar arrangement, blue light may be admitted by the one eye-piece and red by the other, and on the con vergence of the two, a resultant colour, purple, will be observed. This may be termed the binocular vision of colours. It is remarkable that by a mental effort this sensation of a compound colour may be decomposed into its constituents, so that one eye will again see blue and the other red. 6. THE PSYCHICAL RELATIONS OF VISUAL PERCEPTIONS. (V.) General Characters of Visual Perceptions. All visual perceptions, if they last for a sufficient length of time, appear to be external to ourselves, erect, localized in a position in space, and more or less continuous. (d) Visual Sensations are referred to the Exterior. This appears to be due, to a large extent, to habit. Those who have been born blind, on obtaining eye-sight by an opera tion, have imagined objects to be in close proximity to the eye, and have not had the distinct sense of exteriority which most individuals possess. Slowly, and by a process of education, in which the sense of touch played an impor tant part, they gained the knowledge of the external rela tions of objects. Again, phosgenes, when first produced, appear to be in the eye, but when conscious of them, by an effort of imagination, we may transport them into space, although they never appear very far off. (b) Visual Sensations are referred to Erect Objects. Although the images of objects are inverted on the retina we see them erect. The explanation of the effect is that we are conscious not of the imago on the retina, but of the luminous object from which the rays proceed, and we refer the sensation in the direction of these rays. Again, in running the eye over the object, say a tall pole, from base to apex, we are not conscious of the different images on the retina, but of the muscular movements necessary to bring the parts successively on the yellow spot. (c) Visual Sensations are referred to a Position in Spacr.
The localization of a luminous point in space can only be