ILLUMINATION OF OBJECTS; POLARIZED LIGHT 
53 
Fig. 23. 
Position of the Prisms with 
Nicols Crossed. 
The lower nicol placed below the object is called the polarizer; 
the upper nicol, above the object, the analyzer, since it serves 
to examine or analyze the light 
transmitted by the object. For 
the best results a nicol prism 
must be about two and one-half 
times as long as it is thick. A 
long prism for the analyzer is 
cumbersome and undersirable, 
therefore a calcite prism ce¬ 
mented with some resin having 
a different refractive index than 
Canada balsam is generally em¬ 
ployed ; these devices are known 
as Thompson, Gian, Ahrens, etc., prisms after the men invent¬ 
ing them.^ 
Anisotropic crystals so act upon plane polarized light passing 
through them as to resolve the ether vibrations into two com¬ 
ponents polarized at right angles, the planes of vibration of which 
are not coincident with the plane of vibration of the analyzer. 
If a small transparent doubly refracting crystal, or a fragment 
of a transparent anisotropic substance be placed upon the stage 
of the microscope, brought under the cross-hairs of the eyepiece 
and examined between crossed nicols, it will be found that the 
crystal or the fragment becomes alternately bright and dark 
as the stage is rotated. In the bright positions it may even 
become brilliantly colored. The bright and dark positions with 
reference to the cross-hairs together with the presence or absence 
of polarization colors are of great assistance in identifying the 
material being studied. The behavior of crystals under polar¬ 
ized light is discussed in Chapter XI. 
In order to conveniently study the effect of the crystals upon 
the polarized light issuing from the polarizer, it is best that the 
polarizer be so mounted as to permit rotation, and in many 
cases it will be found a great convenience if the mount is pro- 
^ For a very comprehensive description of the various types of prisms, see 
Johannsen, Manual of Petrographic Methods, p. 158. McGraw-Hill, 1914. 
