ILLUMINATION OF OBJECTS; ABBE CONDENSER 
25 
oblique illumination but a black background, and serve an 
entirely different purpose which is discussed elsewhere under 
the head Dark-ground Illumination. In the highest grades of 
microscopes the substage mounting is arranged so as to provide 
a lateral movement of the iris diaphragm by means of rack and 
pinion. Oblique illumination is then obtained by closing the 
diaphragm to a small opening and racking it to one side. 
Oblique illumination is often essential to a proper interpre¬ 
tation of structure and to a sharp differentiation of refractive 
indices. 
The ordinary Abbe condenser is corrected for neither chromatic 
nor for spherical aberration and although it answers all the pur¬ 
poses of illumination in ordinary microscopy with standard objec¬ 
tives, in photomicrography or in combination with objectives 
of the highest grade and in work of the finest kind, its use is 
injudicious. Recourse should be had in such cases to achromatic 
or specially constructed condensers. Since investigations of this 
kind are rare in chemical laboratories, space forbids their con¬ 
sideration. 
In accurate crystallographic studies the microscope condenser 
must be especially free from both chromatic and spherical aber¬ 
ration; and instruments for this class of work are never provided 
with condensers of the Abbe type, but are always fitted with 
light-concentrating devices of special construction. 
It is essential that the optic axis of the condenser shall coincide 
with the optic axis of the microscope, or, in other words, the 
condenser must be accurately centered. In the low-priced micro¬ 
scopes no provision is made for any adjustment of the mount¬ 
ing, the proper position being fixed by the manufacturer. Not 
infrequently through carelessness of workmen and inadequate 
inspection of the finished instrument, microscopes are sold whose 
substage condensers are so badly out of center as to render them 
unfit for high grade work. 
To test the adjustment of an Abbe condenser in a fixed mount¬ 
ing, close its iris diaphragm to the smallest obtainable opening, 
raise the substage as far as it will go; insert a cross-hair eyepiece 
in the body tube and focus with a very low power upon the 
