84 POLARIZED LIGHT AND ITS APPLICATION TO THE MICROSCOPE. 
same time, polarized by refraction in a plane at right angles 
with the plane of the reflected ray. If the light falls npon the 
glass at any other angle, some portion of it will be polarized by 
reflection, and an equal amount will be polarized by refraction, 
but each of the rays thus polarized will be mingled with other 
light that is reflected or transmitted without polarization. 
If instead of transmitting light through a bundle of plates of 
thin glass, we transmit it through only a single plate, and at 
any angle, that plate will polarize a part of the light ; if two 
plates are used, still more light will be polarized, and when a 
sufficient number of plates are used, all the light will be 
polarized. 
Many substances, not perfectly homogeneous throughout, 
have veins, laminae, or isolated spots, capable of producing 
either partial or complete polarization of the light which 
passes through them. 
In Iceland spar the two images produced by double refrac- 
tion are equal in intensity, and are both completely polarized ; 
but a great variety of substances give, by refraction, a secon- 
dary image so faint and so little separated from the ordinary 
image, that its existence is not generally recognized unless 
examined by some test for polarized light. There are many 
substances, both animal and vegetable, that possess this power 
of partial polarization, and which, when viewed by polarized 
light, exhibit an arrangement of structure totally un apprecia- 
ble by ordinary light. 
Even glass which has been cooled more rapidly in one part 
than another, or in which unequal pressure has been exerted 
in any manner, (as by bending or heating on one side,) is 
capable of polarizing some of the light transmitted through it. 
Many crystals found in animal fluids or vegetable tissues, pos- 
sess the power of partially polarizing light. Hence, polarized 
light becomes the most delicate test known for discovering 
differences of density or structure, in animal, vegetable, or min- 
eral substances, and is of great importance as a method of illu- 
mination in microscopic investigations. 
J. & W. GRUNOW & GO’S ILLUSTRATED 
