COMPOSITION OF WHITE LIGHT 



159 



FIG. 152. 



Forms of lenses. 



air becomes more and more dense (c/. 41). A heavenly body is, 

 therefore, never where it seems to be, except when it is at the zenith; 

 that is, directly overhead. 



178. The Lens. A lens is a piece of transparent mate- 

 rial, usually glass, having two curved surfaces, or a curved 

 and a plane surface. Six 

 forms of lenses are shown in 

 Fig. 152. When rays of light 

 pass through a lens, they are 

 bent toward the thickest part 

 of the lens. This is shown in 

 Fig. 153. Parallel rays coming from the left would thus 

 be brought together, or " brought to a focus," at F, on 

 the opposite side of the lens. If, on the other hand, light 

 were produced at the point F, and its rays went through 



the lens, they would be made parallel. 



Focus is from the Latin, and means 



" hearth." 



Burning Glass. The double-convex lens of 



Fig. 153 would bring parallel heat rays, as well 



as sunlight, to a focus at F. It would then be 

 called a " burning glass." Paper, or a match, at F could be set on 

 fire. 



Refraction of Sound. If a lens were constructed of the right mate- 

 rial, it could refract sound waves (cf. 190), and bring them to a focus. 

 The ticking of a watch might then be heard at this focus, although not 

 at places much nearer the watch (Fig. 154). 



179. Composition of White 

 Light. At first thought it may 

 seem likely that white light, such 

 as that of the sun, is simpler in 



FIG. 153. 



The lens brings the par- 

 allel rays to a focus. 



FIG. 154. 



Sound waves can be brought to 

 a focus like light waves. 



