CRYSTALS UNDER THE MICROSCOPE 163 



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: 

 (i) 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 de- 

 grees 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 analyzer 

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

 possible field, 1 replace the analyzer and set both nicols at their 

 zero point. Screen the stage (i.e., the open space between the 

 body tube and stage) and cover the head with a dark cloth. 

 Now observe carefully whether the nicols thus set are in their 

 position of maximum extinction. This is done by turning one 

 of the prisms the least amount possible and noting whether the 

 field becomes darker or lighter. Make a number of observations, 

 closing the eyes for a few seconds each time just before looking 

 into the microscope. 



2. Testing the Cross-hairs. Having adjusted the polarizer 

 and analyzer to the proper position of crossed nicols as ascer- 

 tained above, attach a low power objective, insert a cross-haired 

 eyepiece and place upon the stage previously centered a prepa^ 



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. 



