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Appendix A - Discrimination of Precious Stones

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318                                   Appendix.
polarized ; and the well-known instrument, called a " Nicol's prism," conveniently enables the observer to obtain one of these polarized rays apart from the other,
Pleochroism.
Pleochroism is a term used to express the existence of a plurality of colours in one and the same stone, when viewed by transmitted light under certain conditions. This can be made an invaluable aid in the identification of certain gems, by the help of a little instrument invented many years ago, by the great Austrian mineralogist, Haidinger, and called the Dichroiscope. The mineral lolite, takes its name of Dichroite from its marked dichroism. This mineral and some other stones, such as certain Sapphires and Tourmalines, may show the difference of tint to the unaided eye, but in most cases a special instrument is needed for its detection.
The dichroiscope is a very simple instrument, con­structed as follows :—A cleavage rhombohedron of Iceland spar is fitted into a small metal cylinder, having at one end a sliding cap, perforated through its centre with an aperture, usually square. At the other end is a lens, or combination of lenses, of such focal length that when the sliding cap is pulled out, it will show, in consequence of the double refraction of the Iceland spar, two distinct images of the aperture. If a stone be introduced in front of the aperture, the two images will be of the same or of different hues, according to the optical characters of the stone. Those minerals with crystallize in the Cubic system such as Diamonds, Garnets, and Spinels, show a pair of images identical in colour. But all Precious Stones cryst­allizing in any of the other systems show, except when viewed in the direction of the optic axis, two images, the
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