THE SEKSES. 739 



eye A (fig. 436), with a' of the eye B. 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-fibres correspond with their distribution. The identical points on 

 the upper and lower parts of the retinae may also be shown by the fol- 

 lowing simple experiment. 



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 

 two outer or the two inner sides of both eyes. It is certain, therefore, 

 that neither the upper part of one retina and the lower part of the other 

 are identical, nor the outer lateral parts of the two retinae, nor their 

 inner lateral portions. But if pressure be made with the fingers upon 



A ib ai B 



c 



Fig. 436. Diagram to show the corresponding parts of both retina. 



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 a of the eye A, and upon the inner side a' of the eye B, a 

 single spectrum is produced, and is apparent at the extreme right of the 

 field of vision ; if upon the point b of one eye, and the point b' of the 

 other, a single spectrum is seen to the extreme left. 



The spheres of the two retinae may, therefore, be regarded as lying 

 one over the other, as in c, fig. 436 ; so that the left portion of one eye lies 

 over the identical left portion of the other eye, the right portion of one 

 eye over the identical right portion of the other eye ; and with the 

 upper and lower portions of the two eyes, a lies over ', 1} over #', and c 

 over c'. The points of the one retina intermediate between a and c are 

 again identical with the corresponding points of the other retina between 

 a' and c'; those between b and c of the one retina, with those between b' 

 and d of the other. If the axes of the eyes, A and B (fig. 437), be so 

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



