. Carnegie Institution of Washington publication. COLORED GLASSES. 57 viz, (i) glasses having absorption bands in the visible and in the infra-red spectrum, and (2) glasses having a wide absorption band in the optical region, followed by an increase in transparency with wave-length. The red glasses belong to the latter group, and their behavior is more nearly like that of optically turbid media, the color being due to the presence, in the glass, of small spheres of metal (see Garnett, loc. cit.). To the first group belong the cobalt blue and Schott's blue-violet glass. If the color in some gla
. Carnegie Institution of Washington publication. COLORED GLASSES. 57 viz, (i) glasses having absorption bands in the visible and in the infra-red spectrum, and (2) glasses having a wide absorption band in the optical region, followed by an increase in transparency with wave-length. The red glasses belong to the latter group, and their behavior is more nearly like that of optically turbid media, the color being due to the presence, in the glass, of small spheres of metal (see Garnett, loc. cit.). To the first group belong the cobalt blue and Schott's blue-violet glass. If the color in some glasses is due to the metal, one would expect great opacity in the infra-red, as is known for a thin film of the metal. If the metal is present in a colloidal form, the present examination of colloidal silver would indicate great transparency, with the possibility of small absorption bands in the infra-red. The whole question is in an unsettled state, and an investigation of the transmission of various glasses, blown into fig. 4i- — Black glass. thin films, as described in Carnegie Publication No. 65, page 65, seems highly desirable. In leaving this subject it is of interest to note that a specimen of garnet (Carnegie Publication No. 65, p. 59) was found to have a wide absorption band, with complete opacity extending from to p. This is one of the few substances having a large absorption band near the visible Sphalerite (ZnS). (Cleavage piece; ^= mm.; transparent; slight yellowish tinge; curve c, fig. 40.) This substance was previously examined (Carnegie Publication No. 65, p. 63), using for the purpose a rock-salt prism and a radiometer. In the present examination, the same mirror spectrometer, but a fluorite prism and a bolometer were used. On account of the small dispersion at 1 \x the rock-salt prism is not well adapted for work in this region. The transmission curve of sphalerite appears to have a band at /<. On examination of the original data
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