Ch. 6: Inherent Diamond Qualities & Cutting

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	110	THE DIAMOND

	diamond than in any other. For instance, the indices of refraction for spinel and garnet, which are also, like the diamond, singly refractive stones, are 1.71 and 1.77; that of the diamond is 2.43. To obtain the index of this refraction, draw a line perpendicular to the surface of the body through the point of entry of the ray, and a circle around with this point as a center. A straight horizontal line from the point where the circle intersects the ray, to the perpendicular line, is the sine of the angle of incidence, and the sides of the angle follow the ray and the perpendicular line to the impinging point as a vertex, thereby forming the angle of incidence. The ray on entering the body is bent or refracted toward the perpendicular line. The point in the circle, therefore, within the body, where it is intersected by the refracted ray, would be nearer the perpendicular line, on the other side of it. A line drawn between these two points would be the sine of the angle of refraction and with the sides forming a vertex at the central point with the vertex of the angle of incidence, would be the angle of refraction. It is the comparative lengths of these sines which give the index of refraction. In water it is as 1.33 to 1; in spinel 1.71, in garnet 1.77, and in diamond it is as 2.43 to 1. The light, which falling upon the surface of the diamond is sent flashing on, constitutes the surface brilliancy, and that which finds entry, by the gem's power to hold and return it, forms the internal brilliancy. The light which has entered the stone is now in the grip of more exacting laws. It has lost the full freedom of the air. The denser medium sets bounds,

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