the quality of _refraction_ is due one of the chief charms of certain
precious stones. It is not necessary to explain here what refraction is,
for everyone will be familiar with the refractive property of a
light-beam when passing through a medium denser than atmospheric air. It
will be quite sufficient to say that all the rays are not equal in
refractive power in all substances, so 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 rotated, it
fails _at every point_ to transmit light, which a double-refracting gem
allows to pass except when its optical axis is placed in the axis of the
polariscope, but this will be dealt with more fully when the methods of
testing the stones come to be considered.

_Diaphaneity_, or the power of transmitting light:--some rather fine
trade distinctions are drawn between the stones in this class, technical
distinctions made specially for purposes of classification,