FUNCTION AND PARTS OF THE MICROSCOPE. 15 



(in virtue of which white light is split up into its compo- 

 nents) varied independently of each other. Thus flint glass 

 has only .1 to .2 more refractive power than crown glass 

 with more than twice its dispersive power. If a biconvex 

 lens of crown glass, (C) , be combined with a plano-con- 

 cave lens of flint glass, (i^) , arranged in the opposite sense, 

 as shown in Fig. 13, it is possible so to adjust their oppo- 

 site curvatures that the flint glass shall compensate for 

 all the dispersion caused by the crown glass and neu- 



Fio. 13. — ^The Achromatic Objective. (After Hager-Mez.) 



tralize only half its refraction. This is the principle of 

 the achromatic objective; and through the efforts of 

 Selligues and Chevalier in France, Fraunhofer in Ger- 

 many, Amici in Italy, and Goring, Tulley, and Lister in 

 England, it gradually attained its practical development 

 between 1820 and 183b. With this improvement the 

 compound microscope acquired new importance; and 

 it very shortly developed into one of the most important 

 instruments at the disposal of modern science. 



II. The Immersion Objective. — Lenses made of com- 

 bined crown and flint glass in such fashion as to be 

 achromatic will also show a decreased spherical aberra- 

 tion, and this latter defect may be further reduced to a 

 minimum by adjusting the radii of curvature of the oppo- 



