undergoes another change, known as polarization. Polarized
light is that having vibrations taking place in a single plane instead
of in an innumerable number of planes, as is the case with ordinary
light. A partial polarization of light occurs with every refraction and
reflection, but it is not complete. By means of proper appliances a
perfect polarization can be obtained. Polarized light is of great
advantage in the optical study of gems, since it affords a ray the
plane of whose vibrations can be accurately ascertained. Light may be
polarized by causing it to be reflected from two mirrors, and an
instrument sometimes used for obtaining polarized light is constructed
upon this principle. The most commonly used polarizer, however,
is the so-called Nicol prism, an appliance constructed from two pieces
of Iceland spar, in a manner which can best be understood when the
subject of double refraction has first been considered.
Light
propagated in the anisotropic media previously mentioned, instead of
advancing in all directions with the same velocity, as is the case with
isotropic media, advances in different directions with different
velocities. These directions resolve themselves into two, corresponding
to the directions of the greatest and least elasticity in the medium. A
ray of light entering such a medium is therefore broken up into two
rays, which have distinct properties and move independently of each
other. The refraction of the ray instead of being single, as is the case with isotropic media, is double, and such media are hence said to be doubly refracting. To
this class belong most gems, since substances crystallizing in any of
the systems except the isometric, unless they are amorphous, possess
this property. The most familiar illustration of the double refraction
of light is seen when an object such as a black cross upon paper or a
line of ordinary print is looked at through a piece of Iceland spar.
The characters when thus seen appear double, and of only about half
their normal intensity, except where two images may come together. The
phenomenon is more evident in Iceland spar than in other minerals
because the two rays are more widely separated in this substance than
is usually the case, but the breaking up into two rays and the
separation take place in many other minerals nevertheless. The two
rays are known for distinction as the ordinary and extraordinary rays.
Each is polarized, so that it is evident that if some means can be
found of eliminating one of them, the other may be made to furnish a
convenient source of polarized light. This is what is done in the
construction of the Nicol prism, it being made of two pieces of
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