Light; a course of experimental optics, chiefly with the lantern . at the amount of retardation in a plate ofcrystal thus cut, depends on both the thickness of the plateand the amount of convergence; and that the rings mustbecome closer and more numerous as the plate increases inthickness. The private student will often find a simple tour-maline pincette (Fig. 182) the most convenient slice of tourmaline is mounted so as to be capable ofrotation at each end of the spring tongs, and the crystalplateto be examined is held between them: the rays passingthrough this simple polariscope
Light; a course of experimental optics, chiefly with the lantern . at the amount of retardation in a plate ofcrystal thus cut, depends on both the thickness of the plateand the amount of convergence; and that the rings mustbecome closer and more numerous as the plate increases inthickness. The private student will often find a simple tour-maline pincette (Fig. 182) the most convenient slice of tourmaline is mounted so as to be capable ofrotation at each end of the spring tongs, and the crystalplateto be examined is held between them: the rays passingthrough this simple polariscope into the small pupil of the 3i2 ? ? LIGHT. ••; - [CHAP. eye, are sufficiently corivergenttb exhibit the a single loose tourmalihej such as the one used in therotating frame, held close to the eye with the crystal closeup to it, will show the rings well, if the-whole, and the eye,are turned towards the plane-polairised light from a gla;ssplate or any other polarising surface, or even towardscertain portions of a bright sky (see Chapter XVIL).. Fig. 182.—Tourmaline Pincette. 162. Preparation of CrystalSi—Many crystals can only be prepared, as a rule, by skilled workmen ; and thiemost immense variety, numbering over a hundred, of uni-axials and the K-axials to be next considered, may beobtained from Dr. Steeg and Renter, Homburg von derHohe, who prepare this class of objects for almost thewhole: wqrld. The rarer crystals can hardly, in fact, beobtained elsfewhere; but Messrs. Darker, and otie or two RINGS AND BRUSHES IN CRYSTALS.
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Keywords: ., bookcentury1800, bookdecade1880, bookidcu3192403121, bookyear1882
