CRYSTALS UNDER THE MICROSCOPE 175 



thickness of the crystal. In the position of extinction there is 

 of course no color. 



If the value of the double refraction is known, the thickness 

 of the crystal may be calculated and vice versa. 1 Polarization 

 colors are of greater value in petrological investigations than 

 in chemical analysis. Nevertheless, the analyst should never 

 neglect to note the colors and their intensities when examining 

 preparations between crossed nicols. A valuable clue as to the 

 probable nature of the material under examination may often 

 be thus obtained, since if brilliant polarization colors are seen 

 we may conclude that the substance has a high double refraction 

 and we may thus eliminate from further consideration sub- 

 stances whose double refraction is so weak as to render brilliant 

 interference colors impossible. 



It is often difficult to determine, between crossed nicols alone, 

 whether or not a substance is anisotropic if its double refraction 

 is very weak, and only the faintest tints of gray are produced. 

 Recourse is then had to a selenite test plate cut of such a thickness 

 and orientation that when placed between the nicols with its 

 direction of vibration at 45 degrees to the planes of vibration 

 of the nicols a purple-red interference color is obtained. This 

 particular shade, known as red of the first order, is the most useful 

 of test plate interference colors. When such a test plate is 

 placed either above or below the very weakly polarizing prepa- 

 ration being studied the change of phase in the transmitted 

 light waves is such as to produce a contrasting color. The 

 entire field is colored red ; the polarizing materials or crystals will 

 therefore appear differently colored, according to their thickness, 

 upon a red background. Double refraction so weak as to pass 

 unnoticed will thus be readily recognized. 



The selenite is also most useful in the determination of extinc- 

 tion angles (q.v.), in ascertaining the optical sign + or of 

 biaxial crystals, and in measuring the thickness of thin polarizing 

 rock and crystal sections. 



1 For a full and comprehensive discussion of interference colors and their appli- 

 cation in microscopy the student is referred to Weinschenk-Clark, Petrographic 

 Methods, pp. 73-87, or Johannsen, Petrographic Methods. 



