CRYSTALS UNDER THE MICROSCOPE 177 



them upon the stage of a polarizing microscope, removing the 

 analyzing nicol and rotating the polarizer. If the substance 

 under examination is pleochroic, it will change in color with the 

 rotation of the prism. In the event of the polarizer being fixed 

 and incapable of rotation, rotate the stage. Always carefully 

 shade the preparation with the hand in order to prevent as much 

 as possible confusing reflections. 



If the phenomena observed involve a two-color change the 

 crystals are said to be dichroic; if a three-color change trichroic. 

 Uniaxial crystals can exhibit only a two-color change; biaxial 

 crystals may be trichroic. 



Isotropic crystals possessing a high adsorption power for cer- 

 tain coloring matters may become in the process of their growth 

 highly colored. These crystals, although still retaining their 

 isometric habit are often highly pleochroic. 



Practical application may be made of the phenomenon of 

 pleochroism in differentiating between different textile fibers and 

 different paper fibers stained with certain aniline dyes. Some 

 species of fiber exhibit strong pleochroism and others weak. 



The Measurement of Crystal Angles and Extinction Angles. 

 Since the interfacial angles of crystals of chemical compounds 

 are always constant for similar faces no matter how the compound 

 may have been prepared, it is obvious that angle measurements 

 may often prove of the greatest value in the identification or 

 differentiation of compounds or of crystal systems. When 

 crystals are of sufficient size to be handled determinations of the 

 values of angles by means of some form of goniometer are fraught 

 with no great difficulties, but when the crystals are microscopic 

 and cannot satisfactorily be orientated, the problem becomes 

 exceedingly difficult. 



Fortunately, the chemist is rarely if ever called upon to make 

 very accurate angle measurements ; rapid approximate readings 

 are usually sufficient for analytical work. Moreover, so-called 

 chemical microscopes are incapable of yielding angular measure- 

 ments of the degree of accuracy required in crystallographic 

 investigations. 



Great accuracy on the part of the analyst is seldom essential, 



