17 



its emergence from the prism, its second refraction 

 takes place; it will also be noticed that the light 

 is bent downward or toward the base of the prism 

 and if the prism be imagined reversed with its base 

 upward, the action of the prism on the light will 

 be the same, but its course will be upward, always 

 toivard its base. 



Now a lens in either of its two principle forms is 

 nothing more in effect than the prisms showing 

 as in Fig. 2, where the two bases are placed 



together, how the light is refracted toward the 

 bases and thus converges; or how, as in Fig. 3, 

 where the bases are above and below, the action of 



