REFRACTION AND POLARIZATION. 



6? 



plates of tonrmnline, 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 Nicol 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 vertk-al 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- 



3. 



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 



