VISION. 



459 



ciples of construction, and the mode of operation 

 are exactly the same ; and the only difference 

 is, that the former is an infinitely more perfect 

 instrument than the latter can ever be rendered 

 by the utmost efforts of human art. 



With a view of simplifying the subject, I have assumed, in 

 the account given in the text, that the rays which arrive at the 

 eye are parallel, which in mathematical strictness they never 

 are. The focus of the rays refracted by a convex lens is more 



remote in proportion as the rays are more divergent, or, in other 

 words, proceed from nearer objects. This is illustrated by 

 Figures 412, 413, and 414; to which I shall again have occa- 

 sion to refer in the sequel. 



