VISION. 



531 



DIOPTRICS OP VISION. 



One of the most satisfactory methods of ascertaining that 

 the eye does form images of the objects in the iield of vision 

 is to remove the eye of a recently killed albino rabbit. On 

 holding up before such an eye any small object, as a pair of 

 forceps, it may be readily observed that an inverted image of 

 the object is formed on the back of the eye {fundus). If, 

 however, the lens be removed from such an eye, no image is 

 formed. If the lens be itself held behind the object, an in- 

 verted image will be thrown upon a piece of paper held at a 

 suitable (its focal) distance. By substituting an ordinary bicon- 

 vex lens, the same effect follows. It thus appears, then, that 

 the lens is the essential part of the refracting media, though 

 the aqueous and vitreous humors and the cornea are also focus- 

 ing mechanisms. 



In the actual human eye the focus must correspond with the 

 fovea of the retina if a distinct image is to be formed. 



B 



I II 



Fie. 381.— Refraction by convex lenses (after Flint and Weinhold). The lens may be 

 assumed to consist of a series of lenses (II in figure), for the srfke of simplicity, 

 though, of course, this is not strictly accurate. 



It will appear that we may represent the eye as reduced to 

 the lens and the retina. The experiments referred to above 

 will convince the .student that such is the case. 



