PLEOCHROISM — CRYSTAL ANGLES 
263 
been subjected to chemical treatment. As an example of the 
latter, there may be cited, the change in sign from + to - in 
nitrocellulose as the percent of nitration increases. In nitro¬ 
cellulose low in nitrogen the double refraction is positive, but 
nitrocelluloses high in nitrogen show negative double refraction; 
the change is a gradual one, the transition point being between 
nitrogen contents of ii and 12 per cent.^ It is obvious that 
the polarizing microscope affords a convenient method of ascer¬ 
taining the degree of nitration of a given sample of nitrocellulose. 
Absorption. Pleochroism. — Many compounds have the 
power of absorbing part of the light rays vibrating in certain 
planes and therefore if viewed through the polarizing microscope 
with the analyzer removed will exhibit a change of light intensity, 
in certain positions. This property of crystals known as absorp¬ 
tion should not be confused with a change of color. 
All anisotropic substances to a greater or lesser extent remove 
the rays of certain colors in certain planes from white light sent 
through them. This property when sufficiently pronounced to 
be observable with the normal human eye is termed pleochroism. 
Substances are tested for pleochroism by placing them upon 
the stage of a polarizing microscope, removing the analyzing nicol 
and rotating the polarizer. If the substance -under examination 
is pleochroic, it will change in color with the rotation of the prism. 
In the event of the polarizer being fixed and incapable of rotation, 
rotate the stage. Always carefully shade the preparation with 
the hand in order to prevent as much as possible confusing 
reflections. 
If the phenomena observed involve a two-color change the 
crystals are said to be dichroic; if a three-color change trichroic. 
Uniaxial crystals can exhibit only a two-color change; biaxial 
crystals may be trichroic. 
Isotropic crystals possessing a high adsorption power for cer¬ 
tain coloring matters may become in the process of their growth 
highly colored. These crystals, although still retaining their 
isometric habit are often highly pleochroic. 
1 Chardonnet: Zeit. ang. Ch. 1899, 31. Lunge and Bebie; Zeit. ang. Ch. 1901, 
567. Ambrom: Roll. Zeit. 13 (1913), 200. 
