OPTICAL QUALITIES
Since
the pleasing qualities of gems depend largely upon their effects upon
light, some general statements as to the properties of light, and the
manner in which it is affected in passing through gems, will be
desirable. The generally accepted theory of the transmission of light
is that it moves in a straight line without change of direction in one
and the same homogeneous medium, as vibrations of particles of the
luminiferous ether which may be called light waves, and which take
place at right angles to the direction of transmission. In some media
the velocity of transmission of light is independent of the direction
in which it is propagated. Such media are called isotropic, and
include among gems, opal, diamond, spinel, and garnet. In other media
the velocity of transmission of light varies in different directions.
Such media are said to be anisotropic. Most gems belong to this
class of bodies. The velocity of transmission of light through
different media differs, but has an absolute value for one and the same
substance.
Media
in which light is transmitted at a high velocity are said to be
optically rare, those in which it is transmitted at a low velocity are
said to be optically dense. In passing from one medium to another of
different density, as for instance from air into water, light undergoes
a change in its rate of transmission and a change of direction. This
change constitutes the phenomenon of refraction, the most
familiar illustration of which is seen in the apparent bending of a
stick partly immersed in water. If the amount of this change of
direction be studied, it will be found to have a definite angular value
which is constant for the same substance. Thus, if in the accompanying
figure a ray of light passing through the air from L be supposed to
fall upon the surface of water at A, it will be refracted in the
direction A K. The angle L A B is called the angle of incidence, and K
A C the angle of refraction, B C being a perpendicular to the water's
surface. If from A as a center a circle B C be described, and from the
points m and p where this circle cuts the incident and refracted rays the lines m n and p q be drawn perpendicular to B C, then will
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