Appendix A - Discrimination of Precious Stones

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	Appendix. 315

	medium (as from air into a gem) is bent or refracted towards a line drawn perpendicular to the plane which divides them : and vice versa. b). The sines of the angles of incidence and refraction bear a constant relation to one another for each substance, which relation is known as its refractive index or index of refraction. It is to this property that lenses owe their magnifying power ; the higher the indices, the higher the magnifying power. The refractive index of the Diamond is the highest of any well-known substance. It was the high refractive index of the Diamond that led Newton to suspect its composition, as explained at p. 57. Sir D. Brewster gives the following as the refractive indices (for the yellow ray) of several gem-stones compared with glass :— An instrument called a Reflectometer has been introduced for,the purpose of readily ascertaining the refractive index, and may be conveniently used in the examination of certain gem-stones. Dispersion.

	When a ray of common white light passes through a transparent medium, it may suffer decomposition, and be split up into its component colours·. If the medium be properly shaped, this decomposition of the light is rendered evident, and in place of the white light which entered, there emerges a beautiful group of all the prismatic colours of the rainbow.

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