o black
darkness, it will emit a curious phosphorescent glow for from one to ten
seconds; the purer the stone, the longer, clearer and brighter the
result.

CHAPTER VI.

PHYSICAL PROPERTIES.

D--COLOUR.

Colour is one of the most wonderful effects in nature. It is an
attribute of light and is not a part of the object which appears to be
coloured; though all objects, by their chemical or physical composition,
determine the number and variety of vibrations passed on or returned to
the eye, thus fixing their own individual colours.

We have also seen that if an _equal_ light-beam becomes obstructed in
its passage by some substance which is denser than atmospheric air, it
will become altered in its direction by refraction or reflection, and
polarised, each side or pole having different properties.

Polarised light cannot be made again to pass in a certain direction
through the crystal which has polarised it; nor can it again be
reflected at a particular angle; so that in double-refracting crystals,
these two poles, or polarised beams, are different in colour, some
stones being opaque to one beam but not to the other, whilst some are
opaque to both.

This curious phenomenon, with this brief, though somewhat technical
explanation, shows the cause of many of the great charms in precious
stones, for when viewed at one angle they appear of a definite colour,
whilst at another angle they are just as decided in their colour, which
is then entirely different; and as these angles change as the eye
glances on various facets, the stone assumes a marvellous wealth of the
most brilliant and intense colour of kaleidoscopic variety, even in a
stone which may itself be absolutely clear or colourless to ordinary
light.

Such an effect is called pleochroism, and crystals which show variations
in their colour when viewed from different angles, or by transmitted
light, are called pleochroic, or pleochromatic--from two Greek words
signifying "to colour more." To aid in the examination of this
wonderfully beautiful property possessed by precious stones, a little
instrument has been invented called the dichroscope, its name showing
its Greek derivation, and meaning--"to see colour twice" (twice, colour,
to see). It is often a part of a polariscope; frequently a part also of
the polarising attachment to the microscope, and is so simple and
ingenious as to deserve detailed explanation.

In a small, brass tube is fixed a double-