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 .graves 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, I.e. Johann- 

 sen, Petrographic Methods. 



