POLARIZATION BY REFRACTION. 
73 
era! occurs. A beam of common light, transmitted through 
such a plate, is almost perfectly polarized, and refuses to pass 
through glass and other transparent media, when it falls upon 
them in certain positions. 
The beam of light thus polarized will pass freely through a 
second tourmaline plate, held in the same position as the first, 
as showm at Fig. 37. 
Fig. SY. Fig. 38. 
■k' 
But if one of the plates is rotated before the other, and in a 
plane parallel with it, the light gradually diminishes till the 
two plates are at right angles with each other, as in Fig. 38, 
when the light becomes wholly obscured. As the rotation con- 
tinues the beam gradually reappears, and when half a revolu- 
tion has been performed the light resumes its original intensity. 
This is best illustrated by looking at a candle through the 
tourmaline plates. If one plate only is used, the candle will 
be distinctly seen in every position of the tourmaline. If a 
second plate of tourmaline is held with its axis parallel to the 
axis of the first, the candle will still be seen as before, but if 
one plate is slowly rotated before the other, as described, the 
image of the candle will slowly vanish and reappear alternately 
at every quarter and half revolution of the plate, varying 
through all degrees of brightness to total, or almost total, 
obscurity. These changes depend obviously upon the relative 
position of the plates, and upon the ultimate form or physical 
properties of the crystalline particles of the mineral. 
When the axes of the two plates are parallel, the brightness 
of the image is at its maximum, and when the axes of the sec- 
tions cross at right angles, as in Fig. 38, the image of the 
candle vanishes. 
A ray of light, polarized by reflection, when examined by a 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
