48 
ELEMENTARY CHEMICAL MICROSCOPY 
suitable choice of colors, very remarkable results may be ob¬ 
tained which may greatly facilitate the study of certain sorts of 
material. 
In this connection it may be pertinent to point out that the 
illumination of opaque objects (and transparent objects as well) 
by monochromatic light of different colors often gives informa- 
ation of the greatest value. Colored light may at times reveal 
structures not readily noticed by white light in routine micro¬ 
scopic examinations. In industrial work time and labor are too 
important to be ignored, and if we are dealing with colored 
materials certain colored components of which, are to be dis¬ 
covered, if present, it may happen that we may accomplish our 
ends more rapidly and more easily if we employ yellow, or 
green, or blue, or red light instead of ordinary daylight. 
The color of the background also plays an important part 
when studying objects by reflected light. This is particularly 
true when photomicrographs are to be made. The investigator 
should have at hand small pieces of cards or papers of different 
colors which can be slipped under the preparations to be exam¬ 
ined or photographed. 
Rosenhain and Haughton ^ have recently employed mixed 
color illumination in the study of the crystal structures of alloys 
with excellent results. 
h. By Means of Ultraviolet Light. — When ultraviolet rays 
impinge upon certain substances they become fluorescent and 
glow with violet, red, green or bluish light. The color of the 
fluorescence is peculiar to the substance. Since comparatively 
few bodies exhibit this phenomenon and since the color is a 
further aid in differentiation, advantage has been taken of this 
property of bodies as a means of identification of such sub¬ 
stances not readily recognized when present in low per cents 
in mixtures. To permit the extension of this method to minute 
amounts of material the “ Fluorescence Microscope ” has been 
constructed.^ 
Ordinary glass is practically opaque to ultraviolet rays but 
1 Engineering, 1920, 659. 
^ Made by C. Reichert, Vienna, Austria. 
