780 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
recommended, since by it a strong spherical aberration of the blue rays 
is produced which can in no way be removed by the eye-piece. An 
improvement in the geometrical focus is rather to be sought in the 
opposite direction, viz. by removing the spherical aberration of the blue 
marginal ray by means of the objective, and the resulting magnification 
error by a compensating eye-piece. In this way, if care be taken to 
remove the secondary colour aberration as much as possible, a completely 
colourless image can be obtained. 
It is only possible to simultaneously satisfy many conditions in 
Microscope systems, by keeping the spherical and chromatic corrections 
distinctly separate. In order to be able to do this it is necessary to have 
at one’s disposal a series of glasses with very varied dispersions for the 
Fig. 84. 
same refractive index. For, to correct for chromatic aberration a system 
already spherically corrected, without changing its form, glasses are 
necessary which have approximately the chosen indices and possess the 
dispersions determined by calculation. 
Since the diminution of the aberration of the blue marginal ray 
depends on the strong chromatic over-compensation of the hinder 
members of the system, the question arises whether the middle or the 
upper lens or both are to be over-compensated. Now a strongly over- 
compensated middle lens and under- compensated upper lens lead to a 
system (fig. 83) with slight difference of magnification and strong 
aberration of the blue marginal ray, while a strongly over-compensated 
middle lens and more or less chromatically over-compensated upper lens 
give a system with excessively under-compensated front lens. Thus 
strongly over-compensated middle lenses are to be avoided, and it is best 
