that the middle of the spectrum is generally selected as the mean for indexing purposes.
It
will be seen that the stones in the 1st, or cubic system, show single
refraction, whereas those of all other systems show double refraction ;
thus, light, in passing through their substance, is deviated, part of
it going one way, the other portion going in another direction—that is,
at a slightly different angle—so that this property alone will isolate
readily all gems belonging to the 1st system.
A
well-known simple experiment in physics shows this clearly. A mark on a
card or paper is viewed through a piece of double-refracting spar
(Iceland spar or clear calcite), when the mark is doubled and two
appear. On rotating this rhomb of spar, one of these marks is seen to
revolve round the other, which remains stationary, the moving mark
passing further from the centre in places. When the spar is cut and
used in a certain direction, we see but one mark, and such a position
is called its optical axis.
Polarisation is
when certain crystals possessing double refraction have the power of
changing light, giving it the appearance of poles which have different
properties, and the polariscope is an instrument in which are placed
pieces of double-refracting (Iceland) spar, so that all light passing
through will be polarised.
Since
only crystals possessing the property of double refraction show
polarisation, it follows that those of the 1st, or cubic system—in
which the diamond stands a prominent example—fail to become polarised,
so that when such a stone is placed in the polariscope and
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