PETROGRAPHY AND METALLOGRAPHY. 157 
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 180° 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 go° from the parallel position, no light passes 
and the field of the microscope remains dark. If crystals 
of sodium chloride o1 some other salt crystallizing in the 
