BINOCULAR VISION 



701 



front of and over the other, as in figure 477. Thus, as we have noticed in 

 speaking of the distribution of the optic nerve fibers, the temporal portion 

 of one eye corresponds to or is identical with the nasal portion of the other 

 eye. The upper part of one retina is also identical with the upper part of 

 the other; and the lower parts of the two eyes are identical with each other. 

 The distribution of the optic nerve fibers corresponds with the distribution 

 of the identical points. The identical points on the upper and lower parts 

 of the retinae may also be shown by the following simple experiment. 



FIG. 478. 



FIG. 477. Diagram to Show the Corresponding Parts of the Retinae. 

 FIG. 478. Diagram to Show the Simultaneous Action of the Eyes in Viewing Objects 

 in Different Directions. 



Pressure upon any part of the ball of the eye, so as to affect the retina, 

 produces a luminous circle, seen at the opposite side of the field of vision to 

 that on which the pressure is made. If, now, in a dark room, we press with 

 the finger at the upper part of one eye, and at the lower part of the other, 

 two luminous circles are seen, one above the other; so, also, two figures are 

 seen when pressure is made simultaneously on the outer or the inner sides 

 of both eyes. But if pressure be made with the fingers upon both eyes 

 simultaneously at their lower part, one luminous ring is seen at the middle 

 of the upper part of the field of vision. If the pressure be applied to 

 the upper part of both eyes, a single luminous circle is seen in the middle 

 of the field of vision below. So, also, if we press upon the outer side of one 

 eye and upon the inner side of the other eye, a single luminous spot is pro- 

 duced, and is apparent at the extreme right of the field of vision. The 

 hemispheres of the two retinae may, therefore, be regarded as lying one over 

 the other, as in C, figure 477. If the axes of the eyes, A and B, figure 478, 

 be so directed that they meet at a, an object at a will be seen singly, for 

 the point a of the one retina and a' of the other are identical. So, also, if 

 the object t 3 be so situated that its image falls in both eyes at the same dis- 

 tance from the central point of the retina namely, at b in the one eye 



