Elements of mineralogy, crystallography and blowpipe analysis from a practical standpoint .. . brationsare in the plane of reflection. In (rt) and (B) common light is always present. (<:) By double refraction and total reflection ofone of the rays as in the so-called Nicols prism,made from a cleavage of calcite with a lengthabout twice its thickness, Fig. 349. The two small rhombic faces at 71° to the edge are groundaway and replaced by faces at 68° to the edge. The prism isthen cut through by a plane at right angles both to the new ter-minal faces and to the principal section. The parts ar
Elements of mineralogy, crystallography and blowpipe analysis from a practical standpoint .. . brationsare in the plane of reflection. In (rt) and (B) common light is always present. (<:) By double refraction and total reflection ofone of the rays as in the so-called Nicols prism,made from a cleavage of calcite with a lengthabout twice its thickness, Fig. 349. The two small rhombic faces at 71° to the edge are groundaway and replaced by faces at 68° to the edge. The prism isthen cut through by a plane at right angles both to the new ter-minal faces and to the principal section. The parts are carefullypolished and cemented by Canada balsam, the index of refrac-tion of which is or about that of the extraordinary ray bd,which, therefore, passes through the balsam with but littlechange in direction ; the ordinary ray be, however, with an indexof refraction of , being incident at an angle greater than itscritical angle, is totally reflected. The vibration direction of the emerging lightis parallel to the shorter diagonal of the face of the nicol, as shownby the i6o DESCRIPTIVE MINERALOGY. Fig. II .nfri. s ! • Mirk ((^ By double refraction and absorption. Certain substancesabsorb one ray much more rapidly than the other, hence thicknessescan be chosen for which one ray is totally absorbed, the otherbeing partially transmitted with vibrations all in one plane. Tour-maline is often used, as in this mineral the ordinary ray is muchthe more rapidly absorbed. Polariscope for Parallel Light. The essentials of a polariscope for parallel light are shown inFig- 3 SO- The mirror J/sends parallel rays through the lowerlens X, which concentrates them at the center of Sthe polarizer P; this point is also the focus of the?^ equivalent upper lens X. On emergence from LIII, the rays are again parallel, and undergo double re- ; I I ; fraction in the plate 5 of the substance. At what- ever angle the parallel rays are incident at the plateS, each ray, as AB, F
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