is a term used to express the existence of a plurality of colours in
one and the same stone, when viewed by transmitted light under certain
conditions. This can be made an invaluable aid in the identification of
certain gems, by the help of a little instrument invented many years
ago, by the great Austrian mineralogist, Haidinger, and called the Dichroiscope. The
mineral lolite, takes its name of Dichroite from its marked dichroism.
This mineral and some other stones, such as certain Sapphires and
Tourmalines, may show the difference of tint to the unaided eye, but in
most cases a special instrument is needed for its detection.
dichroiscope is a very simple instrument, constructed as follows :—A
cleavage rhombohedron of Iceland spar is fitted into a small metal
cylinder, having at one end a sliding cap, perforated through its
centre with an aperture, usually square. At the other end is a lens, or
combination of lenses, of such focal length that when the sliding cap
is pulled out, it will show, in consequence of the double refraction of
the Iceland spar, two distinct images of the aperture. If a stone be
introduced in front of the aperture, the two images will be of the same
or of different hues, according to the optical characters of the stone.
Those minerals with crystallize in the Cubic system such as Diamonds,
Garnets, and Spinels, show a pair of images identical in colour. But
all Precious Stones crystallizing in any of the other systems show,
except when viewed in the direction of the optic axis, two images, the