18 



the prisms on the light still remains the same, 

 refracting the rays of light toward the bases, and 

 in this case causing them to diverge or separate. 



If the combinations of prisms be imagined with 

 curved surfaces instead of flat ones, the action of 

 light through them will be precisely the same and 

 we have the two great classes of lenses, converging 

 or convex and diverging or concave, as shown 

 in Fig. 4. 



Fig. 4. 



These two classes are again subdivided, as 

 shown in Fig. 5, showing the three forms which 



Fig. 5. 



are made in each of the convex and concave types 

 and which are called in rotation' as drawn, double 



