162 ELEMENTARY CHEMICAL MICROSCOPY 



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 undesirable, therefore a calcite prism ce- 

 mented with some resin having a different refractive index than 

 Canada balsam is generally employed; these devices are known 

 as Thompson, Glan, Ahrens, etc., prisms after the men inventing 

 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 an anisotropic substance be placed between crossed nicols 

 and slowly turned it will be found that in certain positions some 

 light emerges from the upper nicol while in other positions no 

 light emerges. 



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- 

 vided with a scale graduated to indicate the degree of angular 

 rotation. The analyzer may either slide in and out of the body- 

 tube of the microscope or may fit above and over the eyepiece. 

 The latter style of mounting is often preferable for general chem- 

 ical laboratory work. Analyzers screwing into the body-tube 

 just above the objective are undesirable. 



For convenience the polarizer and analyzer should be so mu- 

 tually arranged that when slipped in place, the position for 

 crossed nicols is at once fixed without the necessity of testing 

 each time for complete extinction of light. 



In the chemical microscope illustrated in Fig. 3, page 20, the 

 mounting of the polarizing nicol is provided with a stud and the 



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

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



