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HAND-BOOK OF PHYSIOLOGY. 



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 appar- 

 ent at the extreme right of the field of vision; if upon the point I 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. 386; 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 



FIG. 388. 



and lower portions of the two eyes, a lies over a', b over V , and c over c'. 

 The points of the one retina intermediate between a and c are again identi- 

 cal with the corresponding points of the other retina between a' and c'\ 

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

 the other. If the axes of the eyes, A and B (Fig. 388), 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 1) in the one eye, and 

 at V in the other,/? will be seen single, for it affects identical parts of 

 the two retinae. 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 of 



