The Astrophysical journal . ent as the plates from Zeiss, No. i*No. 2 illustrates this fact. This crystal was free from flaws. Thereis some hope, therefore, that colored fluorite may yet be foundwhich will serve for prisms and lenses, and which will possess therequisite transparency for use in the region between X 2000 andX 1250. It is well known that the color may be removed from fluorite byheating, and, if the process is carefully carried on, no cracks aredeveloped. This loss of color, however, is accompanied by only thevery slightest gain in transparency, if any. In ten cases out of twelven
The Astrophysical journal . ent as the plates from Zeiss, No. i*No. 2 illustrates this fact. This crystal was free from flaws. Thereis some hope, therefore, that colored fluorite may yet be foundwhich will serve for prisms and lenses, and which will possess therequisite transparency for use in the region between X 2000 andX 1250. It is well known that the color may be removed from fluorite byheating, and, if the process is carefully carried on, no cracks aredeveloped. This loss of color, however, is accompanied by only thevery slightest gain in transparency, if any. In ten cases out of twelveno difference could be noted at all either by the visual or by thephotographic method. A microscopic examination with a power of two hundred diametersshowed little difference in constitution between the transparent West-moreland fluorite and the more opaque colored varieties. In general,the specimens which showed the greater absorption seemed to con-tain the larger number of those fluid-filled cavities which have been - <. •a .a js £ - p 2 ~ £ j OCC tn fe 01 ^ — a ?£UOPh ABSORPTION FOR SHORT WAVE-LENGTHS 51 so often observed. The colorless variety of fluorite from Zeiss wasquite free from these microscopic inclosures. The fluorescence excited by cathode rays was somewhat less bril-liant in the colorless than in the colored fluorite, but the differenceswere not of such a magnitude as to give an indication of the relativetransparencies of the specimens. Topaz.—Next to the colored fluorites, topaz from Ceylon showsthe greatest transparency of all the substances examined. The resultobtained by the photographic method is shown in spectrum No. specimen was mm thick; it is thus inferior to quartz. Topazfrom Japan, Utah, and Siberia is much less transparent than thatfrom Ceylon. It is possible, however, that this difference may beconsidered rather as a peculiarity of the individual specimen than asa distinctive property connected with a region. Gypsum.—This subst
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectspectru, bookyear1895
