in
the second decimal place, while in the case of zircon specimens of the
different varieties have furnished, for the ordinary refractive index
with yellow light, figures ranging between 1.95 and 1.84, that is a
difference of 0.11. The unit to which all these indices are referred is
that of air, which is taken as 1.0. In connection with the refraction
of light by precious stones mention should be made of the phenomenon of
total internal reflection. This, so conspicuous in the case of diamond
and other gems of high refrangibility, fills the stone with light and
contributes very largely to the beauty of its appearance. This subject
is fully discussed in Part I. of the volume on Precious Stones by Dr.
Max Bauer and Mr. L. J. Spencer.
Dispersion of Light.—When
a ray of light passing from one medium to another is bent or refracted,
the light being composite and consisting of rays having different
degrees of refrangibility, it suffers dispersion as well as refraction.
In this way the several component rays, differently coloured, are
separated more or less widely from each other, and are said to be dispersed. Upon
this property of gems depends that peculiar quality of " fire "—the
play of prismatic hues, which is the most marked characteristic of the
diamond. It is the difference between the extreme indices of refraction
of the red ray and of the violet ray at the ends of the visible
spectrum. It is best measured by taking as standards certain fixed
lines in the solar spectrum ; but for the purpose of comparing the
dispersive powers of different stones the following list of approximate
dispersion-coefficients (quoted from M. Jannettaz) will perhaps suffice
:
