SIXGLE VISIOX. 



66 7 



In quadrupeds, the relation between the identical and 

 non-identical parts of the retinae cannot be the same as in 

 man; for the axes 



of their eyes gene- Fi ^' l89 ' 



rally 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 

 it, the image of the 

 object must fall, in 

 both eyes, on the 

 outer portion of the 

 retinae. Thus tjie 

 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 dis- 

 tinct and single vision of objects, & or c, the points b' and &", 

 or c' c" , in the two retinae, on which the images of these 

 objects fall, must be identical. All points of the retina in 

 each eye which receive rays of light from lateral objects 

 only, can have no corresponding identical points in the 

 retina of the other eye; for otherwise two objects, one 

 situated to the right and the other to the left, would appear 

 to lie in the same spot of the field of vision. It is probable, 

 therefore, that there are, in the eyes of animals, parts of 

 the retinae which are identical, and parts which are not 

 identical, i.e., parts in one which have no corresponding 

 parts in the other eye. And the relation of the two retina? 

 to each other in the field of vision may be represented as 

 in fig. 190. 



The cause of the impressions on the identical points of 

 the two retinae giving rise to but one sensation, and the 

 perception of a single image, must either lie in the striic- 



