238 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



Instead of a single lens, such a microscope is often fitted with 

 a magnifying glass formed of two lenses, separated by a diaphragm, in 

 order to destroy the spherical aberration and to secure achromatism. 



FIG. 174. Simple microscopes. 



Such a combination is Wollaston's doublet.. Fig. 175 represents 

 doublet with improvements by Ch. Chevalier. 



FIG. 175. Simple microscope with doublet. Wollaston's doublet, improved by Chevalier. 



The compound microscope fulfils this object : it possesses magni- 

 fying glasses with convex lenses of different powers and fields, which 

 may be used at pleasure. 



