THE SENSES. 621 



placed exactly in front of, and over the other (as in Fig. 417, c). Thus, 

 the outer lateral portion of one eye corresponds to, or, to use a better 

 term, is identical with the inner portion of the other eye; or a of the e} r e 

 A (Fig. 418), 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. 



This is proved by a 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 pres- 

 sure 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 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 V 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. 417; 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 ', ~b 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 f and c' of the other. If the axes of the eye, A and B (Fig. 419), 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 fi be so situated that its image falls in both eyes at the same 

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

 eye, and V in the other, ft will be seen single, for it affects identical 

 parts of the two retinas. The same will apply to the object y. 



In quadrupeds, the relation between the identical and non-identical 

 parts of the retina cannot be the same as in man; for the axes of their 

 eyes generally diverge, and can never be made to meet in one point of 

 an object. When an animal regards an object situated directly in front 



