. 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. 50 GENERAL CHARACTERS OF PRECIOUS STONES double images of the needle. Contrary to the previous ease, this experiment must be performed in a lighted room. When a stone thus examined shows unmistakably double images the fact may be regarded as a decisive proof of the doubly refracting nature of the stone ; when, however, single images only are observ
. 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. 50 GENERAL CHARACTERS OF PRECIOUS STONES double images of the needle. Contrary to the previous ease, this experiment must be performed in a lighted room. When a stone thus examined shows unmistakably double images the fact may be regarded as a decisive proof of the doubly refracting nature of the stone ; when, however, single images only are observed the stone cannot be stated to be singly refracting on these grounds alone, for stones which have only feeble double refraction may give double images so close together, or may be overlapping, that to recognise the double character of such images is a matter of considerable difficulty. The investigation by the direct method of the kind of refraction possessed by a stone thus requires a certain amount of skill, which is only acquired by practice. On this account the re- fraction of stones is often investigated by an indirect method, which has the advantage of being applicable to stones with rounded surfaces, and also to small and irregular fragments of material, neither of which could be used with the method of direct observation. Further, very small cut stones ai-e easily examined by the indirect method, while their examination by the direct method would present difficulties. The instrument used for the indirect observa- tion of the singly or doubly refracting character of a stone is known as the polariscope. A simple form of this instrument, sufficient for the present purpose, is shown, one-third' the actual size, in Fig. 27. This consists of a wooden box, H, into the cover, pp, of which fits the circular object-carrier, I 00; the latter consists of a plate of glass in a -o , . i X. i- â n , brass setting, and may be easily rotated. From Polariscope for oDservation in paral
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