all
minerals crystallising in the hexagonal and tetragonal systems have one
such axis, and it coincides with the principal axis of the crystal.
Minerals of the remaining crystallographic systems (that is, the
orthorhombic, the monosymmetric or monoclinic, and the triclinic) have
two such axes; all crystals with double refraction are anisotropic.
The
property of double refraction can be made use of in the identification
of certain gem stones. If on looking through a cut gem we see two
images of an object through one pair of facets, we know that the gem is
a doubly-refracting one ; but if we cannot be sure of seeing two, we
cannot say at once that it is singly refracting, as there are some
possible fallacies; firstly, we may be looking along the optical axis
of a uniaxial crystal, or along one of the axes of a biaxial crystal,
in either of which cases only one image would appear; secondly, the
separation of the two images may be too slight to be observed by the
unaided eye.
c. Now,
if a ray of white light be reflected from the surface of glass at a
certain angle, a particular set of the light undulations are suppressed
and the remaining undulations can no longer be reflected in all
directions; if these residual vibrations be received on another plate
of glass inclined to the rays at the same angle, but with the plane of
incidence at right angles to the plane of incidence on the first piece
of glass, no light is reflected from the second glass. If now between
the two pieces of glass a crystal belonging to the cubic system, or an
amorphous transparent body, be placed, there is still no light
reflected to the eye. Should, however, an anisotropic body be
interposed, light will be transmitted to the eye again, unless
perchance the optic axis coincides with the direction
