668 



THE SENSE OF SIGHT. 



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- 



rally diverge, and 

 \ft 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 the 

 image of the object 

 a (fig. 1 84) 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, I or c, the points I 1 and J", 

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

 jects 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, there- 

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

 retinae which are identical, and parts which are not iden- 

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

 in the other eye. And the relation of the two retinae to 

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

 fig. 1 83. 



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 struc- 



