122 ZOOLOGY. 



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, 

 which being refracted at f, will become a i. By this refrac- 

 tion, rays of light passing through the lens meet at last in a 

 focus. 



When the surface on which the rays strike is concave instead 

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

 by figure 79, 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 a d and a i will 

 assume the direction of cz/and a g. 



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 principles to vision. When the rays 

 of light fall on the cornea, some are reflected, and this gives 

 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 Cowards 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 

 reflected ; those falling on the pupil pass directly through it, 

 which of course admits more or less light, according as it is 

 more or less contracted. Under a strong light the pupil 

 diminishes; with a feeble light it dilates. The rays now 



