HOW PICTUEES AKE FORMED BY LENSES 



461 



Fig. 217.) Now, each ray is supposed to be made up of 

 vibrations of a substance which fills all space and is called 

 ether. The rate at which this ether vibrates determines 

 the color of the light ray. These vibrating lines or rays 

 when they pass from one transparent substance into 

 another of different density are bent in definite directions, 

 the shape and density of the transparent body determin- 

 ing the direction of bending. Glass structures called 

 lenses are simply masses of transparent matter so shaped 

 as to control the direction of the bending in rays which 

 pass from air into them. By means of them it is possible 



I'lG-. 218 Effect of lenses on rays of light; A, a concave lens spreading the rays; 



£, a convex lens bending the rays to a point. 



to concentrate the rays which pass through them (con- 

 vex lenses) or to spread the rays (concave lenses). (See 

 Fig. 218.) 



The point to which rays are converged by a convex lens, 

 or from which they seem to be spread by a concave lens, 

 is called the focus of those rays and the action of the 

 lens is called focussing. By its shape, a convex lens is 

 able to focus the rays which it receives from a luminous 

 body, so that the rays, which come from a certain point on 

 the object shall, after passing through it, all converge at a 

 certain spot. In this way the lens is able so to collect the 



