. 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. DOUBLE REFRACTION OF LIGHT 47 rarity and costliness. It is also possible by this means to distinguish glass imitations, which are always singly refracting, from genuine precious stones, which are for the most part doubly refracting. The kind of refraction, single or double, exhibited by a body is a necessary consequence of the ciystalline structure
. 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. DOUBLE REFRACTION OF LIGHT 47 rarity and costliness. It is also possible by this means to distinguish glass imitations, which are always singly refracting, from genuine precious stones, which are for the most part doubly refracting. The kind of refraction, single or double, exhibited by a body is a necessary consequence of the ciystalline structure of its substance, and varies in the different crystal systems. All amorphous bodies, together with all those which crystallise in the cubic system, are singly refracting, while all other crystals, without exception, namely, those included in the hexagonal, tetragonal, rhombic, monoclinic, and triclinic systems are doubly refracting. It, is thus possible fi-om the behaviour of a stone with respect to the refraction of light ta learn whether, on the one hand, it is amorphous or crystallises in the cubic system, or whether, on the other hand, it crystallises in one of the five remaining crystal systems; and this observation can be made on a very small irregular fragment of the mineral. Thus in the example just quoted we know that the singly refracting spinel must crystallise in the cubic system, while the doubly refracting ruby crystallises in one of the remaining five systems, namely, the hexagonal. Since the observation of the kind of refraction, whether single or double, exhibited by a stone is a step towards determining to which of the crystal systems it belongs, and more- over is frequently a decisive test of its identity, it is important to be acquainted with the method of making this observation. In the third part of this book, dealing specially with the determination of precious stones, considerable use will be made of this method, and it will also be mentioned under the
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