72 
POLAKIZED LIGHT AND ITS APPLICATION TO THE MICEOSCOPE. 
Other positions. It also loses the property of penetrating trans- 
parent bodies in particular positions. Light, so modified, is 
said to be Polarized. 
Light may be polarized by reflection y'mTTi any polished sttr- 
face., at an angle peculiar to each substance, e. g., from glass at 
56° 45', from water at 53° 11', rock crystal 56° 58', diamond 
68 ° 1 '. 
From a very extensive series of experiments, made to deter- 
mine the maximum polarizing angles of various bodies. Sir 
David Brewster arrived at the following law. The index ofre- 
f raction is the tangent of the angle of polarization. It follows. 
therefore, as a geometrical consequence, that the reflected 
polarized ray forms a right angle with the refracted ray. It 
is generally known that only a part of the light falling upon a 
polished surface is reflected ; the remainder is either dispersed or / 
absorbed, or, if the body is transparent, it is transmitted. How / 
as the differently colored rays, of which ordinary white light is 
composed, are not refracted equally by any transparent medium, 
it follows that the index of refraction, and consequently the 
polarizing angle, varies for the differently colored rays, there- 
fore a ray of white light will not be perfectly polarized by re- 
flection at any angle, but there is a certain range of angle 
within which the polarization is more or less perfect, a portion 
of light remaining unpolarized even at the angle of most per- 
fect polarization. 
97. Polarization by refraction. Certain crystallized j 
minerals and salts have the property of polarizing light trans- 
mitted through them, owing probably to some physical pecu- / 
liarity in the form or crystalline arrangement of their ultimate 
molecules. Such are tourmaline, calc spar, (Iceland spar,) / 
quartz, and also many artificial salts, of which sulphate of j 
iodine and quinine (Ilerapathite) is the most remarkable./ 
The transparent brownish varieties of tourmaline furnish T ' 
most convenient polarizing plates. For this purpose the to 
maline crystal is cut into plates about of an inch in thii 
ness and polished, the plane of section being parallel to t 
vertical axis of the hexagonal prism, in which form this ni 
J. & W. GRUNOW & GO’S ILLUSTRATED 
