SINGLE VISION. 523 



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 produced 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 luminous object 

 be such that similar images of the same object fall on identi- 

 cal parts of the two retinse, 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 /? 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, /9will be seen single, for it affects identical 

 parts of the two retinse. The same will apply to the object /. 



In quadrupeds, the relation between the identical and non- 

 identical parts of the retinse cannot be the same as in man ; 



FIG. 189. 



for the axes of their eyes generally diverge, and can never be 

 made to meet in one point of an object. When an animal re- 

 gards an object situated directly in front of it, the image of 

 the object must fall, in both eyes, on the outer portion of the 

 retiuse. Thus the image of the object a (Fig. 191) will fall at 

 a' in one, and at a" in the other : and these points a' and a" 

 must be identical. So, also, for distinct and single vision of 

 objects, b or c, the points b' and b", or c' c", in the two retinse, 



