54 ELEMENTARY CHEMICAL MICROSCOPY 



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 

 mutually 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. 25, page 62, the 

 mounting of the polarizing nicol is provided with a stud and 

 the substage ring into which the polarizer fits has a notch into 

 which this stud fits. The analyzer mounting is notched and 

 the draw-tube of the microscope has a tiny projecting pin at St 

 over which the notch slips. When working with instruments of 

 this type, always see that the studs or pins are seated as deeply 

 into the notches as they will go, then set the graduations of both 

 polarizer and analyzer at zero; this will give crossed nicols and 

 a field of maximum darkness. 



The analyst should always subject his instrument to a search- 

 ing examination and satisfy himself that it is properly con- 

 structed and that any measurements obtained will be accurate 

 and reliable. The most important points to be ascertained are: 

 (1) whether, when the graduated circles of polarizer and ana- 

 lyzer are each set at zero, the nicols are exactly crossed; (2) 

 whether the directions of the cross-hairs of the oculars lie 90 

 degrees apart and correspond to the planes of vibration of the 

 crossed nicols; and (3) whether the graduations on the rotating 

 circles of polarizer and analyzer are equivalent and correspond 

 to the graduations on the circumference of the stage. 



1. Testing for Properly Crossed Nicols. - - Remove the ana- 

 lyzer and objective. Set the plane mirror so as to yield the 

 brightest possible field, 1 replace the analyzer and set both nicols 



1 High grade petrographic and crystallographic microscopes are tested for 

 properly crossed nicols by pointing them directly at the sun. See Wright, F. E. 

 Petrographic Methods, 1. c, p. 62. 



