STRUCTURE OF THE EYE. 281 



brain. With a little trouble, a real eye, say that of an ox, 

 can be dissected out, and employed as a camera obscura, the 

 operator seeing in the back of the eye, or " retina/' the same 

 image which the ox would have seen if it had been alive. 



In photography, the operator, when he has found that 

 a perfect image is thrown upon the ground glass, which 

 represents the retina of the eye, substitutes for it a sensi- 

 tive surface, on which the rays are projected, and which, 

 by chemical means, produce a permanent instead of a fleeting 

 object. 



EXAMPLES of other lenses may be found in Nature. She, 

 moreover, can perform a task which man has never even 

 attempted, namely, the change of form in a lens according to 

 the duty which it has to do. How this wonderful object is 

 attained we shall presently see. 



There is a form of lens extremely useful in Optics, namely, 

 the " Plano-convex" lens. This is, in fact, one half of a double 



HUMAN EYE : SECTION OF CORNEA, &C. PLANO-CONVEX LENS. 



convex lens, the section being made through its edges, and the 

 plane sides polished as well as the convex. As, however, this 

 is only a half of the double convex lens, it does not need 

 further explanation. Its natural counterpart may be seen in 

 the annexed illustration. 



A somewhat more complicated form of lens is called the 

 " Meniscus/' one side of which is convex and the other concave. 

 A good example of the meniscus may be found in the old- 

 fashioned watch-glass, before watchmakers took to flattening 

 them, and watch- wearers were not ashamed to carry a " turnip/' 

 in which there was room to spare for the works. If a section 

 of such a glass were taken, it would assume the form of a half- 



