founded
with an appearance of transmitted light. A doubly refracting stone
will, however, when inserted, be lighted internally, showing much the
same color, though less strongly, as that which it possesses in
ordinary light. On revolving the stone by means of the movable stage,
it will be seen to become dark four times during a complete revolution,
the intervals of darkness occurring every 90° from each other.
Thus
quartz may be distinguished from diamond, quartz from glass, zircon
from diamond, or any doubly refracting stone from a glass imitation,
and so on. Diamond cannot be distinguished from glass, nor from spinel,
by this test, since all are singly refracting. Other tests, such as
those of specific gravity and hardness, will, however, be sufficient to
distinguish in such cases. It is of course true that stones cut from
doubly refracting minerals in certain directions appear like singly
refracting ones, and a possible error may be made on this account. In
practice, however, the likelihood of meeting with stones cut in just
such directions is very small, and may be ignored. An apparatus
constructed for the determination of gems by the above methods is
illustrated in the following figure (p. 32).* Here the polarization of
the light below the stone is accomplished by means of two mirrors, and
thus the cost of one Nicol prism is saved. The stone is placed upon the
stage d. The light, polarized by the mirrors, passes through
the stone into the tube above containing the analyzer, and through this
to the eye. By rotating the tube in the holder /, the distinction in
appearance between singly and doubly refracting minerals can be readily
seen. The ordinary petrographical microscope also affords the
necessary appliances for determinations of this kind. Tourmaline tongs
furnish another combination of a polarizer and analyzer, but they
allow too little light to pass through to be of practical value for
determining minerals.
Doubly
refracting substances have another feature in distinction from singly
refracting ones, in the fact that the rays passing through them are
differently absorbed, and hence give different colors in several
directions, while singly refracting substances are normally of the same
color in all directions. In the degree to which they exhibit this
property of dichroism, or pleochroism, as it is called, minerals vary.
Iolite is one of the most strongly dichroic minerals, and can plainly
be seen to be dark blue in one direction and clove-brown in another.
Transparent zircon is often pinkish brown in color when looked at it in
the direction of the vertical axis, and asparagus-green when seen
laterally. Tourmaline is often nearly opaque when looked at in the
direction of the vertical axis, but transparent when
* This instrument can be obtained of R. Fuess, Steglitz bei Berlin, Germany, at a cost of eighteen to twenty dollars.
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