DEVICES FOR SEEING 



295 



1.25 instead of 1.5, then the 

 radius of the second arc would 

 be taken as 1.25 inches. 



If the ray of light were to 

 strike the glass surface at a 

 small angle, as the ray kb, it 

 would be refracted back into 

 the glass if it could get out. 

 At such an angle it is, there- 

 fore, totally reflected at b to I. 



Some simple experiments 

 with any convex lens like a 

 magnifying glass or a reading 

 glass will help make clear some 

 things that it is necessary to 

 understand in order to com- 

 prehend the working of cam- 

 eras, microscopes, telescopes, 

 or other instruments using 

 lenses. If you hold such a 

 lens so that the rays of sun- 

 light will strike it squarely, 

 the light is brought to a single 

 point on any surface such as a 

 sheet of paper held at the 

 proper distance from the lens 

 (Fig. 142). The sun is so far 

 away that the rays of light 

 entering the lens are practi- 

 cally parallel. The point at 

 which these rays meet is 

 known as the focus of the lens, 

 and the distance from that 

 point to the lens is the focal 

 length of the lens. To be 



FIG. 

 a focus 

 glass. 



142. The beam of light brought to 

 by a plano-convex lens, or burning 



