THE SELENITE PLATE 261 



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 thick- 

 ness 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 use- 

 ful 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 thick- 

 ness, 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 polariz- 

 ing rock and crystal sections. 



One of the best examples of the every-day practical applica- 

 tion of the polarizing microscope and selenite plate by chemists 

 is in the differentiation of pure fresh butter from very old, or 



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. 



