SENSE OF SIGHT. 



205 



A, a solid oblong piece of glass ; a, a, a stream of light, which 

 strikes the surface of the glass, b, c, and which, being perpendicular, 

 suffers it to pass through without refraction. 



24. Now let us take a concave lens, and see what effect 

 that will have on the light. 



Fig. 11. 



A, a solid oblong piece of glass; a, a, a stream of light which strikes 

 the surface of the glass 6, c, and vyhich being concave, causes the light 

 in its passage through it to diverge. 



25. Now try a convex lens, such as the crystalline humour 

 of the eye, and see how that will affect the rays of light. 



We see that this form of glass causes the rays of light 

 18 



