Elementary chemical microscopy elementarychemi00cham Year: 1921 52 ELEMENTARY CHEMICAL MICROSCOPY A nicol prism consists of a long rhomb of calcitc cut length- wise in an oblique plane forming angles of 90 degrees with the upper and lower faces of the rhombs and cemented together again with Canada balsam, see Fig. 22. If a ray of light R enters such a prism it is polarized, being resolved into two component rays vibrating at right angles to each other. One of these rays O, known as the ordinary ray is deflected slightly more than the other and strikes the balsam cement at such an angle as to
Elementary chemical microscopy elementarychemi00cham Year: 1921 52 ELEMENTARY CHEMICAL MICROSCOPY A nicol prism consists of a long rhomb of calcitc cut length- wise in an oblique plane forming angles of 90 degrees with the upper and lower faces of the rhombs and cemented together again with Canada balsam, see Fig. 22. If a ray of light R enters such a prism it is polarized, being resolved into two component rays vibrating at right angles to each other. One of these rays O, known as the ordinary ray is deflected slightly more than the other and strikes the balsam cement at such an angle as to be totally reflected; the other ray called the extraordinary ray, passes through the prism and emerges completely polarized. In the diagram at S is shown a cross-section of the rhomb. The direction vb through a shorter diameter of the prism rhomb is the plane or direction of vibration of the< nicol. If, after emerging from the first prism, the extraordinary ray be sent into a second nicol so placed that its plane of vibration is coincident with or parallel to the direction vb of the first, the ray emerges parallel to its entrance direction at R. In this position the nicols are said to be parallel. But if the second nicol be turned through 90 degrees, thus taking a position such that its plane of vibration intersects that of the first at 90 degrees, the extraordinary ray will behave as though it were the ordinary ray and is completely turned aside. No light emerges from the upper nicol. In this position the nicols are said to be crossed, see Fig. 23. The arrows indicate the planes of vibration in the direction of the short Fig. 22. Construction and Path of Light Rays in a Nicol Prism. 1 In the newer polarizing microscopes, the prisms often do not have a rhombic cross-section and therefore their planes of vibration do not fall in the direction of a short diagonal. The position of the planes of vibration must then be ascertained experimentally; see Weinschenk, Da
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