colour 33
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 lie absolutely clear or colourless to ordinary light.
Such
an effect is called pleoehroism, and crystals which show variations in
their colour when viewed from different angles, or by transmitted
light, are called pleo-chroic, 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-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
