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[Goettle W. <cen...@im...>]

The lustre
of glass, quartz, etc.; some experts subdividing this for greater
defining
accuracy into the "sub-vitreous" or lower type, for all but perfect
specimens. ([zeta]) Metallic or Sub-metallic. The former when the lustre
is perfect as in gold; the latter when the stones possess the less true
lustre of copper. ~Colour.~--Colour is an effect entirely dependent
upon light, for in the total absence of light, such
as in black darkness, objects are altogether invisible to
the normal human eye. In daylight, also, certain objects reflect so few
vibrations of light,
or none, that they appear

grey, black, or jet-black; whilst
those which reflect all the rays of which light is composed, and in the
same number of vibrations, appear white. Between these two extremes of
_none_ and _all_ we find a wonderful play
and variety of colour, as some gems allow the red rays only
to pass and therefore appear red; others allow the blue rays only and
these appear blue, and so on, through
all the shades, combinations and varieties of the colours of which light
is composed, as revealed by the prism. But this is so important a matter
that it demands a chapter to itself. The third physical property

of light, PHOSPHORESCENCE,

is the

property possessed
by certain gems and minerals of becoming phosphorescent on being

rubbed, or on having their temperature raised by this or other means. It
is difficult to say exactly whether this is due to