Iceland
spar cut in definite directions, and cemented together by Canada
balsam. The construction is shown in the accompanying figure. The
parallelogram represents the outline of the prism, and the line running
nearly as a diagonal shows where the two parts are joined together.
A ray of light, m n, falling upon the prism is at once refracted into two rays, n 0 and n E. The ray n 0 upon reaching the layer of balsam is totally reflected, and passes out at 0,, where it disappears. The ray n E,
however, passes through, and reaches the eye as a ray of polarized
light, having its vibrations in a single plane. If now this ray fall
upon another Nicol prism standing in the same vertical direction, and
similarly oriented, it can pass through without sensible loss or
change, and so on through a large number if necessary. If, however, the
second prism while maintaining the same vertical direction be rotated
90' about its vertical axis, the ray upon reaching it will follow the
path n 0 instead of n E, since the ordinary and
extraordinary rays are situated at right angles to each other. It will
therefore be absorbed and lost, and no light will reach the eye.
If
a plate of a singly refracting substance be interposed between the two
prisms, no change will occur in the above-named phenomena; but if a
doubly refracting mineral be inserted instead, the field of view will
light up, except in four positions, 90° from each other. It is obvious
that the passage of light through the second prism in the latter case
comes from the fact that the polarized light from the first prism is
broken up into two rays in traversing the doubly refracting plate, one
of which is traveling in such a direction as will permit it to pass
through the second prism. These differences of behavior of doubly
refracting as compared with singly refracting bodies afford a
convenient and accurate means of distinguishing gems, for the tests
can be made without danger of injury to the stones. The essential
features of an apparatus for the purpose are two Nicol prisms set in a
frame one above the other, with a stage, preferably a revolving one,
between. One of the prisms must be capable of being rotated about its
axis. The lower prism is usually called the polarizer; the upper one
the analyzer. Having turned the prisms with reference to each other so
that the field of view is dark, when a singly refracting substance,
such as diamond, spinel, garnet, or glass, is inserted between the two,
no lighting up of the field can be observed except such as may come
from a reflection from facets of the object. This reflection should not
be con-
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