Light; a course of experimental optics, chiefly with the lantern . 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 A Calcite, analyser crosteAi, D
Light; a course of experimental optics, chiefly with the lantern . 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 A Calcite, analyser crosteAi, D Nitre, shovring Tioth „ anaJyeer parallel. E ,r sVgMfy -ratateiL. C Single Aaeie of HrpoM. F „ aoees rotated- 4-5°. XV.] PREPARING CRYSTALS. 313 Other English opticians prepare calcite, quartz, borax, sugar,and some dozen others of those most usually inquired crjstals, however, are quite within the reach, of theamateur. Nitrate of soda, if a good clear crystal can befound, is a fine uni-axial. A slice may be ground onground glass, polished partially with a little water, andmounted in Canada ba;lsam between two glass circles. Or,in winter, a piece of clear ice, J inch or so thick, held (for itcan hardly be placed in the stage) in front of the barenozzle, with the Nicol held in front and rotated, will givebeautiful rings. Ferro-cyanide of potassium or prussiate ofpotash, is a cheap crystal, found in prisms or tablets, andeasily splits across the optic axis in slices, which havenatural polished faces, and therefore need no other pre
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