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. 4 i- 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 1.2 to 2.6 p. This is one of 

 the few substances having a large absorption band near the visible spectrum. 



Sphalerite (ZnS). 

 (Cleavage piece; ^=1.53 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 1.8 /<. On 

 examination of the original data and curves, it appears that in redrawing 

 the curve for publication the draftsman placed the point at 1.3 (i a little 

 too high (should read 59 per cent), which in the reduced illustration makes 

 a depression at 1.8 p.. 



The curve obtained with the fluorite prism is given in fig. 40, and 

 represents conditions more accurately. A Nernst glower was used as a 

 source, which gave large deflections in this region of the spectrum. 



The transmission curve increases uniformly from 47 per cent at 0.6 fi 

 to 56 per cent at 1 p., then decreases uniformly to 54 per cent at 



