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 x 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. 2 



Ordinary glass is practically opaque to ultraviolet rays but 



1 Engineering, 1920, 659. 



1 Made by C. Reichert, Vienna, Austria. 



