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PRECIOUS STONES.
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in
the substance. They can be distinguished under the polariscope by the
feeble change from light to dark as they are rotated and by the light
not being uniform over the whole field of view.
Another
important phenomenon arising from double refraction remains to be
considered. In an anisotropic substance the vibrations composing the
one ray are at right angles to those composing the other ray. Now, a
different absorption of light occurs in these two planes, and as a
consequence images of different colours are seen; this is known as
pleochroism. For the observation of these colours an instrument known
as a dichroscope is used ; it consists of a cleaved piece of Iceland
Spar mounted in a tube having a lens at one end and a square aperture
at the other. The lens can be adjusted to give focussed images of the
aperture, and at the end where the aperture is a carrier to hold the
gem is fitted so that it can be rotated around the axis of the tube. A
singly-refracting gem placed over the square aperture gives two images
of exactly the same colour; but a coloured doubly-refracting stone
gives two images of distinctly different colours. Care must be taken to
move the gem so as to ensure viewing in other than an optic axis.
C.
Phosphorescence is seen in some minerals after exposure to light. For
example, a Diamond that has been exposed to sunlight and then taken
into a dark room gives off a soft light. The phenomenon is not only
produced by the action of light, for some minerals show it on being
rubbed (e.g., Diamond and Agate), while others show it on being heated (e.g., Topaz). Fluor Spar, Aragonite, and Kunzite also phosphoresce strongly.
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