78 



SPECIAL SENSES. 



A circle is supposed to be equivalent to a polygon with an 

 infinite number of sides. A regular double-convex lens is a 

 transparent body bounded by portions of a sphere, and may 



FIG. 6. 



be assumed to be composed of an infinite number of prisms. 

 The action of a convex lens is to converge the rays of light 



FIG. 7. 



falling upon different portions of their surface so that they 

 cross at a certain distance behind the lens. If we imagine 

 the lens A B (Fig. 7) to be free from spherical aberration, 



