260 
ELEMENTARY CHEMICAL MICROSCOPY 
In order to observe the interference figures with the chemical 
microscope, place the condensing lenses above the polarizing 
nicol, center the crystal or crystal section. Use a i or i inch 
or 4-millimeter objective. Focus the preparation and light well. 
Remove the eyepiece, place the analyzer in its proper position 
upon the top of the microscope tube, cross the nicols and look 
into the instrument. The interference figure will appear as a 
tiny image situated far below the eye. Petrographic and crys¬ 
tallographic microscopes are generally provided with a specially 
constructed lens which slides into the microscope tube above 
the analyzer and below the eyepiece. With this device (Ber¬ 
trand lens) the interference figure is greatly enlarged and it is 
unnecessary to remove the ocular, but in all instruments without 
this special device and where the analyzer fits above the ocular, 
the ocular must be removed in order that the interference figure 
shall be visible. 
Interference, or axial figures as they are also sometimes called, 
must not be confused with the black cross observed in spheru- 
lites and starch granules placed between crossed nicols. 
Interference or Polarization Colors. The Selenite Plate. — 
As stated above, when light enters an anisotropic crystal it is 
polarized or resolved into two rays vibrating at right angles to 
each other. These two rays are propagated at different veloci¬ 
ties, hence one component is slightly retarded and upon emerging 
from the crystal one ray is slightly behind the other in rate of 
vibration; they are, therefore, vibrating in a different phase. If 
the crystal lies between crossed nicols, these rays upon enter¬ 
ing the analyzer are again split, and owing to the difference of 
phase the waves interfere and color results. Hence the crystal 
will appear more or less colored. The brilliancy of color will 
depend upon the character (strength) of double refraction and 
the 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 
Moses, the Characters of Crystals, N. Y., 1899. Luquer, Minerals in Rock Sec¬ 
tions, N. Y., 1898. F. E. Wright, Petrographic Methods, Chapter V, l.c. Johann- 
sen. Petrographic Methods. 
