precisely the same and we have the two great classes 

 of lenses, converging or convex and diverging or 

 concave, as shown in Fig. 5. 



Fig. 4. 



Fig. 5. 



These two classes are again subdivided, Fig. 6 

 showing the three forms of the convex and concave 



IV 



Fig. 6. 



types which are, beginning at the left: 

 / double convex, II plane convex, III convex 

 meniscus, IV double concave, V plane concave, 

 VI concave meniscus. 



