of the prism on the light will be the same, but the 

 light will be bent upward, always toward the base. 



Now, a lens in either of its two principal forms is in 

 effect a combination of two prisms. In Fig. 2, where 



the two bases are 

 placed together, is 

 shown how the light 

 is refracted toward 

 the bases and thereby 

 converged. In Fig. 3, 

 where the bases are 

 in reverse position, the 



fig. 2. 



action of the prisms 



on the light is the same, the rays being refracted 

 toward the bases thus causing them to diverge or 

 separate. 



In Fig. 4 is 

 shown how, by in- 

 creasing the number 

 of prism faces, the 

 form of a lens is 

 gradually approach- 

 ed. 



If the combina- 

 tions of prisms be 

 imagined with curved surfaces instead of flat ones, 

 their action on light passing through them will be 



Fig. 3. 



