by
the stone being put in front of this square, when the two colours are
seen quite distinctly.. Not only is this a simple means of judging
colour, but it enables a stone to be classified readily. For if the
dichroscope shows two images of the same colour, then it may
possibly be a carbuncle, or a diamond, as the case may be—for
single-refracting stones, of the first or cubic system, show two images
of the same colour. But if these two colours are different,
then it must be a double-refracting stone, and according to the
particular colours seen, so is the stone classified, for each stone has
its own identical colour or colours when viewed through this small but
useful instrument.
How
clear and distinct are these changes may be viewed without it in
substances strongly dichroic ; for instance, if common mica is viewed
in one direction, it is transparent as polished plate-glass, whilst at
another angle, it is totally opaque. Chloride of palladium also is
blood-red when viewed parallel to its axis, and transversely, it is a
remarkably bright green. The beryl also, is sea-green one way and a
beautiful blue another ; the yellow chrysoberyl is brown one way and
yellow with a greenish cast when viewed another way. The pink topaz
shows rose-colour in one direction and yellow in another. These are
perhaps the most striking examples, and are mostly self-evident to the
naked eye, whilst in other cases, the changes are so delicate that the
instrument must be used to give certainty ; some again show changes of
colour as the stone is revolved in the dichroscope, or the instrument
revolved round the stone.
Some stones, such as the opal, split up the light-beams
