11*'. ZOOLOGY. 



The form of the surface of the body on which the light 

 tails, modifies ^ivatly the further direction of the ray. Thus, 

 let us suppose that three rays start from the same point, a 

 (Fig. ()('), traverse the air, and fall on the surface of a convex 

 lens. The ray " r striking the surface of the lens perpendicularly 

 will pass directly through it, experiencing no deviation ; but 

 the ray a d will be refracted at e, and proceed towards 

 the perpendicular ef; the same will happen to the ray a g, 

 whieh being refracted at f, will become a i. By this re- 

 fraction, rays of light passing through the lens, meet at last 

 in a locus. 



When the surface on which the rays strike is concave instead 

 of convex, the opposite effect takes place, as may be understood 

 by figure 67, in which b b represents the concave surface 

 a, the point whence the three rays start, a c the perpendicular 

 ray passing directly through, and the rays ad and a i will 

 assume the direction of af and a g. 



b e J 



The deviation which the rays of light thus experience is 

 proportionate to the convexity of the lens ; and the degree of 

 refraction is also in the ratio of the density of the body and 

 its combustibility. 



233. Apply these principlesto vision. When the ray.- 

 of light fall on the cornea, a part are reflected, and this ^ives 

 to the eyes their brilliancy and the power of seeing objects 

 reflected by them, as in a looking-glass. The rays passing 

 through the dense cornea are refracted towards the perpen- 

 dicular ; they now meet the aqueous humour, which being less 

 refrangible than the cornea, restores them somewhat to their 

 primitive direction. Thus a greater number of rays pass 

 through the pupil than could have happened by any other 

 arrangement. The rays which strike the iris are absorbed or 



