suitable flint glass with about one-half the refracting 

 angle and in reversed position is placed close to the 

 first or crown glass prism the red ray will be refracted 

 in the direction r r r" , and the violet in the direction 

 7/ z/', so that on emerging at r" and v" respectively, 

 the red and blue rays will continue in a parallel course 

 and thus be recomposed into a beam of white light, 

 which is changed in its direction from a b d to r" e. 



Thus it is possible not only to avoid dispersion but 

 to obtain a converging effect. If now the prisms will 

 be imagined as a combination of lenses 

 we have a so-called corrected or achro- 

 matic combination or lens. 



If the chromatic and spherical aberra- 

 tions are both corrected it is called 

 aplanatic. The convex lens is made of 

 crown glass and the concave <& flint glass. 



The corrected lens shown in Fig. 13 

 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. 



Fig. 13. 



