SINGLE VISION. IDENTICAL RETINAL POINTS. 871 



If the head is fixed, an idea of the form of the common field of vision can be 

 obtained by alternately closing one eye, and turning the other inward. It will 

 then be seen that the field is pear-shaped, broad above, narrow below, and that 

 the profile of the nose cuts out a portion corresponding to its size, between the 

 upper broader portion and the lower narrow part. If a pasteboard card be held 

 upright close to the face, the outline of the common field may be traced upon it 

 with a pen. 



SINGLE VISION. IDENTICAL RETINAL POINTS. 



HOROPTER. SUPPRESSION OF DOUBLE IMAGES. 



If the retinas of both eyes be considered as a pair of concave saucers 

 placed one within the other, so that the two yellow spots, and the corre- 

 sponding quadrants of the retinas coincide, all those points that corre- 

 spond are called identical or corresponding points. The two meridians 

 that separate the corresponding quadrants are known as lines of separa- 

 tion. The identical points are characterized physiologically by the fact 

 that when light acts upon them at the same time, the stimulation is 

 referred by a psychical act to one and the same place in the visual field 

 (in a direction through the nodal point of each eye). The stimulation 

 of the two identical points of the retina produces, therefore, only one 

 image in the field. Hence, all of those objects of the external world, 

 the rays from which pass through the nodal points to identical points 

 of the retina, are seen singly, because their images are referred to the 

 same part of the visual field, so that they coincide. Double images are 

 produced by all other objects, whose images do not fall on identical 

 portions of the retina. 



The proof for what has been said is readily provided. If a linear object with 

 the points 1,2,3 (Fig- 303) be looked at with both eyes, the corresponding points 

 of the retinal images will be i, 2, 3 and 3, 2, i, which are obviously identical 

 (corresponding) points on the two retinas. If there is, at the same time, a point 

 (A) closer to the eye, or another point (B) farther from the eye, and the eyes are 

 directed toward the object i, 2, 3, the visual rays from neither A (A a, A a) nor 

 B (B b, B b) will fall upon identical retinal points: therefore double images of 

 A and B appear. 



The following simple experiment also is instructive. If a point of ink (for 

 example 2) upon white paper be looked at, the image will fall in both foveas (2, 2), 

 which are of course identical points. By pressing laterally on one eye, so that it is 

 somewhat displaced, two points immediately appear, because the image of the 

 point no longer falls on the fovea of the displaced eye, but on an adjoining, not 

 identical, point. Similarly in intentional squinting all objects appear to be double. 



The vertical lines of division of the retinas do not coincide exactly with the 

 vertical meridians; but exhibit a slight divergence above (from 0.5 to 3), which 

 varies in amount in different individuals, and even in the same individual 

 at different times, while the horizontal lines of division coincide. Images that 

 fall upon the vertical lines of division appear to be perpendicular to those on the 

 horizontal, although they are in reality not so. Therefore, the vertical lines of 

 division are the pseudovertical meridians. 



Some investigators consider the identical points of the retina a congenital 

 arrangement, while others think that they are acquired by ordinary use. In- 

 dividuals who squint from birth have, however, single vision; under such cir- 

 cumstances the identical points must be arranged differently. 



Horopter is the term used to indicate the aggregate of all those points in 

 space, from which rays, entering both eyes, held in any given position, meet at 

 identical points of the retinas. It varies for the different positions of the eyes. 



i. In the primary position of both eyes, when the visual axes are parallel, 

 rays drawn from two identical points of the two retinas are also parallel and 

 intersect only at an infinite distance. The horopter for the primary position is, 

 therefore, a vertical plane at an infinite distance. 



