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, (F), 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- 
Fic. 13.—THEe AcHRomatic Osjective. (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 Iialy, and Goring, Tulley, and Lister in 
England, it gradually attained its practical development 
between 1820 and 1830. 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. 
11. 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- 
