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:
| Diamond | ·044 | Red garnet | ·027 | |
| Borosilicate of lead | ·049 | Tourmaline | ·019 | |
| Flint glass | .036 | Crown glass | ·015 | |
| Blue sapphire | ·029 | Rock crystal | ·014 |
Polarization of Light.—There are several ways in which light may suffer the remarkable change known as polarization. If we
