92 A POPULAR TREATISE ON GEMS
easily
seen in the above apparatus by interposing a thir: plate of gypsum or
mica between the two mirrors. When the interposed plate belongs to'a
monoaxial mineral, there is seen in the second mirror a system of
circular concentric colored rings intersected by a black cross (fig.
97). If the mineral is binaxial, one or two systems of elliptical
colored rings appear, each intersected by a black stripe (fig. 98). In
certain cases this stripe is curved, or the two systems of rings unite
in a lemniscoidal form (fig. 99). When the planes of polarization are
parallel, the black cross and stripe appear white (fig. 100), showing
that in this direction the crystals act like singly-refracting
minerals. Quartz, again, in close relation to its system of
-crystallization, exhibits a circular polarization of splendid
prismatic colors, which, on turning the plate, change in each point in
the order of the spectrum, from red to yellow, green, and blue. In
order to produce these changes, however, in some specimens the plate
must-be turned to the right, in others to the left, showing a
difference in the Crystalline structure.
Pleochroism.—Closely
connected with double refraction is that property of transparent
minerals named pleochroism (many-colored), in consequence of which they
exhibit distinct colors when viewed by transmitted light in different
directions. Crystals of the tesseral system do not show this property;
"while in those of the other systems it appears in more or less
perfection; and in the tetragonal and hexagonal minerals as dichroism
(two colors), in the rhombic and clinohedric systems as trichroism
(three colors). In most cases these changes of color are not very
decided, and appear rather as different tints or shades than as dis-
