SR ae ee gs 
REFRACTION AND POLARIZATION. 67 
plates of tourmaline, cut from a crystal parallel to the vertical 
axis, also are used for this purpose. Another method of ob- 
taining it is by reflection—light, when reflected at a certain 
angle from a polished surface, being polarized; the angle of 
reflection differs for different substances. 

The above figure represents a simple polariscope made with 
two tourmaline plates, which is convenient for many ordinary 
observations. The best instruments for the purpose are made 
with Nico] prisms, and are adapted to microscopic work. The 
prisms, placed within the tube of the instrument, one of them 
below the stage, are arranged so as to admit of revolution; and 
the stage also has a graduated circle and revolves. The com- 
pound microscope also is often converted into a polariscope by 
Nicol prisms arranged for this purpose. 
When a crystal with one axis of polarization, as, for example, 
calcite, is examined by means of a ray of polarized light passed in 
the direction of the vertical axis, concentric circular rings are 
seen, having the colors of the spectrum intersected by either a 
black or a white cross, as in figs. 1, 2. To make the observa- 






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tion it is necessary that the calcite crystal should have its ex- 
tremities polished at right angles to the vertical axis. If a 
tourmaline plate be placed against or near one of its polished 
