ILLUMINATION OF OBJECTS; ABBE CONDENSER 
27 
and the amount of the fault will be indicated by the magnitude 
of image displacement. 
In the better grades of Abbe condensers the mounting is fitted 
with two centering screws, which permit moving the entire con¬ 
denser so that the optic axis of the condenser lenses becomes 
coincident with the optic axis of objective and ocular. 
The simplest method for easily centering adjustable Abbe 
condensers is to have a cap made, fitting exactly over the top 
lens of the condenser; at the exact center of this cap an exceed¬ 
ingly tiny hole is drilled falling in the optic axis of the apparatus. 
The microscope is focused upon this hole, illuminated by the 
light transmitted by the condenser and the bright spot seen is 
brought by means of the centering screws so that its center is 
coincident with the center of the field. 
It is the rule to always use the plane mirror with the Abbe 
condenser; but when the windows of a laboratory have small 
panes or wide cross bars it is often impossible to properly illu¬ 
minate an object with the plane mirror and Abbe condenser 
without projecting an image of the window bars into the field. 
Either the microscope must be moved very close to the window 
or the concave mirror must be used; the latter plan necessi¬ 
tates closing the iris diaphragm two-thirds or more and lowering 
the condenser. In aggravated cases a disk of ground glass may 
be placed below the condenser or in front of the mirror. The 
use of a disk of thin, fine ground glass below the condenser will 
in fact be found a distinct gain in ordinary practice in the illu¬ 
mination of most objects. By its use softer, clearer and more 
easily interpreted images will often be obtained and the true 
colors of objects will be more easily recognized. 
The ring attached to the lower part of the condenser and 
arranged to swing aside serves to carry disks of blue glass to be 
employed when working with artificial light. By this means a 
much less fatiguing illumination is obtained, and providing the 
proper intensity of blue glass is at hand, white light giving 
proper color values is secured. Blue glass should always be 
placed below the condenser when working with yellow artificial 
lights. Most manufacturers supply blue glass disks with all 
