666 THE SENSE OF SIGHT. 



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

 upper and lower portions of the two eyes, a lies over a , b 

 over &', and c over c'. The points of the one retina inter- 

 mediate 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 c of the other. In short, all other parts 

 are non-identical : and, when they are excited to action, 

 the effect is the same as if the impressions were made on 

 different parts of the same retina : and the double images 

 belonging to the eyes A and B, are seen at exactly the same 

 distance from each other as exists between the image of 

 the eye A and the part of the retina of the eye A which 

 corresponds to, or is identical with, the seat of the second 

 image in the eye B ; or, to return to the figure already 

 used in illustration (fig. 1 8 8), if a of one eye be affected, 

 and b' of the other, the distances of the two images a and b' 

 will, inasmuch as a is identical with a , and b' with b, lie at 

 exactly the same distance from each other as images pro- 

 duced by impressions on the points a b of the one eye, or a! 

 b' of the other. 



In application of these results to the phenomena of 

 vision, if the position of the eyes with regard to a lumi- 

 nous object 'be such that similar images of the same 

 object fall on identical parts of the two retina), as occurs 

 when the axes meet in some one point, the object is seen 

 single ; if otherwise, as in the various forms of squinting, 

 two images are formed, and double vision results. If the axes 

 of the eyes, A and B (fig. 189), 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 /3 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 at b ' in the other, ft 

 will be seen single, for it affects identical parts of the two 

 retine. The same will apply to the object y '. 



