OF SIGHT. 



67 



by the eves, I, in the accompanying diagram (fig. 43), falls upon 

 the two terminal points of the two eyes' axes, a and b. The 

 points in the two eyes, A and B, which correspond or are 

 similarly situated, with reference to all surrounding points 



are entitled IDENTICAL, inasmuch as they comport themselves 

 subjectively as if they were in reality but a single point, and 

 images impressed upon them excite in the mind the idea of 

 but one image. Besides these, there are other points of the 

 retina which are also identical or correspondent ; in other 

 words, which present single mental conceptions of double 

 retinal impressions ; but it is a law that the objects and cor- 

 responding points of the retina must lie in a certain circle, 



