. Precious stones, a popular account of their characters, occurrence and applications, with an introduction to their determination, for mineralogists, lapidaries, jewellers, etc. With an appendix on pearls and coral. Precious stones; Pearls; Corals. 40 GENERAL CHARACTERS OF PRECIOUS STONES. Fig. 11. Total reflection. the angle of incidence J CZ> becomes greater and greater so the angle of refraction 5C£ also becomes greater and greater. When the angle of incidence reaches a certain value, represented by A^CD, the corresponding angle of refraction B^CE will be a right-angle ; the refracted r
. Precious stones, a popular account of their characters, occurrence and applications, with an introduction to their determination, for mineralogists, lapidaries, jewellers, etc. With an appendix on pearls and coral. Precious stones; Pearls; Corals. 40 GENERAL CHARACTERS OF PRECIOUS STONES. Fig. 11. Total reflection. the angle of incidence J CZ> becomes greater and greater so the angle of refraction 5C£ also becomes greater and greater. When the angle of incidence reaches a certain value, represented by A^CD, the corresponding angle of refraction B^CE will be a right-angle ; the refracted ray will then emerge from the stone in a direction parallel to the bounding surface MN. Obviously at 90° the angle of refi-action has reached its maximum value and no further increase is possible. Should the angle of inci- dence now be increased, even by a small amount, it will then be impossible for the ray of light to leave the stone, and it will be refracted no longer, but simply reflected by the bounding surface back into the stone. In Fig. 11, the incident ray Afi is reflected from the surface MN, along the line CB^ inside the stone. This takes place according to the usual laws of reflection, the angle of incidence AfiD being equal to the angle of reflection BfiD. In the same way, every ray incident upon the surface MN at a greater angle than AfiD, will be unable to pass out of the stone, and will be reflected back again by the surface MN; AJOB^, for example, is the path of such a ray and its reflection. When light, travelling in one medium, as, for example, air, strikes the surface of a denser medium, such as a precious stone, a portion of it enters the stone and is refracted as described above, while the remaining portion is reflected from the surface. This takes place invariably, whatever may be the angle at which the incident light strikes the surface of the denser medium. In the reverse case, when light travelling in one medium, for example a precious stone, strikes
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