assume that a beam of ordinary or natural light, freely traversing any medium, has what we may call identical properties on all its sides, then,
should that beam encounter any obstacle, as by reflection or
refraction, it exhibits to a greater or less extent different
properties on different sides—is, in fact, polarized. One quality of
this polarized light is that it cannot be again reflected at a certain
angle, nor can it again traverse in a certain direction the crystal in
which it has suffered this change. But the amateur of precious stones
is mainly concerned with these two facts, that in some doubly
refracting crystalline minerals the two oppositely polarized beams are
of different colours ; and, secondly, that some transparent gem-stones
are more or less opaque, in one direction at least, to one of the two
oppositely polarized beams. Thus it will be clear that upon double
refraction and its concomitant polarization depends that property of
many gems which is known as pleochroism, and which may be mostly easily
recognised by that useful little instrument, the dichroiscope.
Pleochroism.—When
a distinctly coloured precious stone is examined by means of a
dichroiscope it will invariably show two images of the same hue or of
different hues. Should the two images of the square opening of the
instrument be identical in colour,'then the specimen may be a garnet, a
spinel, or a diamond ; it cannot be a ruby, a topaz, or a beryl, all of
which show twin colours differing in a perfectly recognizable degree
from each other. However, before proceeding with the description of the
special applications of the dichroiscope, a word on the construction of
the instrument may be introduced. It consists of a cleavage
rhombohedron of Iceland spar, having its longer edges nearly an inch
long and its shorter edges about three tenths of an inch each. In the
original form of the instrument a small glass prism of 18° was cemented
on each of the small end faces of the prism ; but this may be done away
with if these end faces be ground and polished so as to be
perpendicular to the length of the prism.
