PETROGRAPHY AND METALLOGRAPHY. I57 



the ordinary ray from plane polarized light and allowing 

 only the extraordinary ray to pass. 



For studying polarization phenomena, the microscope 

 is provided with two Nicol prisms, properly mounted, 

 one placed below the stage and known as the polarizer, 

 the other inserted, as a rule, in the tube of the microscope 

 above the objective, and known as the analyzer. One 

 or both of the Nicols and the stage of the microscope 

 are arranged to be rotated at will. The polarizer breaks 

 up the light from the mirror into two rays of plane polarized 

 light, and of these suppresses the ordinary ray. The 

 remaining extraordinary ray passes up through the 

 stage and objective of the microscope to the analyzer. 

 When the analyzer occupies two positions relative to the 

 polarizer, one in which the oblique cut surfaces of the 

 two are parallel, and another at i8o° from this, it is as if 

 they were continuous, and the ray of plane polarized light 

 passes freely. At the intermediate points, 90° from the 

 parallel position, no light passes at all, since the extraor- 

 dinary ray bears to the second prism the same relation 

 which the ordinary ray does to the first, and is therefore 

 suppressed. At intermediate points the ray is broken up 

 into two components, with vibrations at right angles to 

 each other, and one of these components passes the 

 analyzer, producing a sort of twilight, the intensity of 

 which increases as the parallel position is approached. 



If, now, the polarizer and analyzer be crossed, placed, 

 that is, at 90° from the parallel position, no light passes 

 and the field of the microscope remains dark. If crystals 

 of sodium chloride 01 some other salt crystaUizing in the 



