27 



aplanatic. The convex lens is made of crown glass 

 and the concave of flint glass. 



Fig. 11. 



The corrected lens shown in Fig. 11 is an 

 achromatic lens of the simplest form and while not 

 absolutely corrected is generally used when the 

 demands on it are not too great. Better correction 

 is obtained when two or more of these lenses are 

 used in combination and they are thus used in 

 some of the lenses of the compound microscope. 

 The variety of forms, due to the variety of glass 

 from which combinations may be made, is almost 

 infinite. 



