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Ch. 8: Optical Qualities

Ch. 8: Optical Qualities Page of 252 Ch. 8: Optical Qualities Text size:minus plus Restore normal size   Mail page  Print this page
seen laterally. In other doubly refracting minerals the dichroism may not be sufficiently apparent to be positively observed with the naked eye. The detection of the dichroic character can usually be made, however, with the aid of the little instrument known as the dichroscope.* This consists of an oblong rhombohedron of Iceland spar with a glass prism
of 18° cemented to each extremity, or with the end faces ground and polished so as to be perpendicular to the length of the prism. It is placed in a metallic cylindrical case, and is provided with a convex lens at one end, and a square hole at the other, the focal length of the lens being such as to show a distinct image of the square opening. On look-
*This instrument can be obtained of Negretti & Zambra, 38 Holborn Viaduct, London, at a cost of about five dollars.
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Ch. 8: Optical Qualities Page of 252 Ch. 8: Optical Qualities
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