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
