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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
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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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