A popular guide to minerals : with chapters on the Bement Collection of minerals in the American Museum of Natural History, and the development of mineralogy . e made by polarized light and suchpolarized light may be parallel or convergent and it is usually pro-duced by the passage of ordinary light through a polarizer by whichthe light is polarized. Above such a polarizer in the tube of amicroscope is placed another polarizer which however in this posi-tion is called an analyzer: and between the polarizer and the an-alyzer the thin section under examination is placed (Fig. 271)upon a stage. T
A popular guide to minerals : with chapters on the Bement Collection of minerals in the American Museum of Natural History, and the development of mineralogy . e made by polarized light and suchpolarized light may be parallel or convergent and it is usually pro-duced by the passage of ordinary light through a polarizer by whichthe light is polarized. Above such a polarizer in the tube of amicroscope is placed another polarizer which however in this posi-tion is called an analyzer: and between the polarizer and the an-alyzer the thin section under examination is placed (Fig. 271)upon a stage. The polarizer and analyzer consist of nicol prismswhich are made of cleavage rhombohedrons of calcite, three timesas long as broad; these are cut as in the oblique plane in Fig. 272,and the halves cemented together with Canada balsam^ and, theends are ground plane. This combination insures a ray of polar-ized light, [two polarized rays, by double refraction^ are formed,but one (the ordinary ray) is reflected from the surface of theCanada balsam and absorbed by the blackened side of the prism,the other (the extraordinary ray) passes through] for the exam-. ination of the crystal sections. Above this polarizer is the analyzei,similarly constructed. When the light passes through the polar-izer it is completely extinguished by the analyzer, when the shorterdiagonal of the analyzer is placed at right angles to the shorterdiagonal of the polarizer, or the nieols are crossed. But this ex-tinction is replaced by light passing through, when a crystal sec-tion is inserted between polarizer and analyzer, if the crystal is notisometric. Glass and isometric crystals remain darkened, and nolight emerges at the analyzer, but generally with sections from allother systems light is seen, while the nicols remain crossed. And 74 A POPULAR GUIDE TO MINERALS this obviously is the crystal section is itself double refract-ing and twists, so to speak, the light emerging through the polarizer,so
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectmineral, bookyear1912
