THE SELENITE PLATE 
261 
of the crystal may be calculated and vice versad 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 
' 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. 
