VISION. 



325 



all intents, a camera obscura: for in both these instrumenls, 

 the objects, the principles of consti^ction, and the mode of 

 operation are exactly the same; and tlie only difference is, 

 that the former is an infinitely more perfect instrument thart 

 the latter can ever be rendered by the utmost efforts of hu- 

 man 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 Fi- 

 gures 412, 413, and 414j to which I shall again have occasion to refer in the 

 sequel. 



§ 3. Structure of the Eye, 



One of the many points of superiority which the eye pos- 

 sesses over the ordinary camera obscura is derived from its 

 spherical shape, adapting the retina to receive every portion 

 of the images produced by refraction, which are themselves 

 curved: whereas, had they been received on a plane surface, 

 as they usually are in the camera obscura, a considerable 

 portion of the image would have been indistinct. This sphe- 

 rical form is preserved by means of the firm membranes 

 which protect the eye, and which are termed its Coats; and 

 the transparent media which they enclose, and which effect 



