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Appendix A - Discrimination of Precious Stones

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Appendix.
This act of splitting up is called the Dispersion of light. It is the phenomenon familiar to all in a chandelier drop. As might be anticipated, the dispersion is highest in the Diamond ; in fact, the dispersive power of this stone is more than three times as great as that of rock crystal. It is upon this property that the matchless quality called " fire " in the Diamond depends. The lower the dis­persive power, the less fire in the gem ; the higher the dispersive power, the more brilliant and iridescent is the fire which it reflects from its surfaces. The dispersive power of certain kinds of glass or paste may be very high, but their softness renders them comparatively worthless for ornamental purposes.
Double Refraction.
The Diamond, Spinel, Garnet, and all other substances crystallizing in the Isometric or Cubic System, or those occurring in the amorphous condition, normally possess only simple refraction. The rest of the gems which crys­tallize in systems other than the cubic, are said to exhibit double refraction ; that is to say, when a ray of light passes through them, it is split up into two rays, one of which— called the ordinary ray—follows the laws of refraction just described, while the other—called the extraordinary ray—follows a totally different law. This splitting or dividing of the ray depends upon the direction in which the light is transmitted through the gem ; there being a certain position in which the ray suffers no division, and the substance simply acts as an ordinary medium, possess­ing single refraction ; this direction is called the axis of no refraction or the optic axis. On looking at a small
Appendix A - Discrimination of Precious Stones Page of 366 Appendix A - Discrimination of Precious Stones
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