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, Glan, Ahrens, etc., prisms after the men invent- 

 ing them. 1 



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- 



1 For a very comprehensive description of the various types of prisms, see 

 Johannsen, Manual of Petrographic Methods, p. 158. McGraw-Hill, 1914. 



