LENSES 



117 



emergent ray is parallel to the incident ray. But when a 



beam of light passes through a triangular glass prism, such 



as a chandelier crystal, its direction is greatly changed, and 



an object viewed through a 



prism is seen quite out of its 



true position. 



Whenever lightpassesthrough 



a prism, it is bent toward the 



base of the prism, or toward the 



thick portion of the prism, and 



emerges from the prism in quite 



a different direction from that in 



which it entered (Fig. 68). Hence, when an object is looked 



at through a prism, it is seen quite 

 out of place. In Figure 68, the 

 candle seems to be at S, while in 

 reality it is at A. 



112. Lenses. If two prisms are 

 arranged as in Figure 69, and 



FIG. 68. When looked at through 

 the prism, A seems to be at 6". 



FIG. 69. Rays of light are con- 

 verged and focused at f. 



two parallel rays of light fall upon the prisms, the beam A 



will be bent downward toward the thickened portion of the 



prism, and the beam B will be bent 



upward toward the thick portion of 



the prism, and after passing through 



the prism the two rays will intersect 



at some point F, called a focus. 



If two prisms are arranged as in 

 Figure 70, the ray A will be re- 

 fracted upward toward the thick end, and the ray B will be 

 refracted downward toward the thick end ; the two rays, on 

 emerging, will therefore be widely separated and will not 

 intersect. 



Lenses are very similar to prisms ; indeed, two prisms 



FIG. 70. Rays of light are di- 

 verged and do not come to any 

 real focus. 



