image prism of calcite or
Iceland spar, which has been achromatised--that is, clear, devoid of
colour--and is therefore capable of transmitting light without showing
any prismatic effect, or allowing the least trace of any except the
clear light-beam to pass through. At one end of this tube there is a
tiny square hole, the opposite end carrying a small convex lens, of such
a strength or focus as to show the square hole in true focus, that is,
with perfectly sharp definition, even up to the corners of the square.
On looking through the tube, the square hole is duplicated, two squares
being seen. The colours of a gem are tested 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 as does a
prism, and show a wonderful exhibition of prismatic colour, which is
technically known as a "play of colour." The descriptive term
"opalesce