ON THE OPACITY OF CERTAIN GLASSES FOR 

 THE ULTRA-VIOLET. 



By Louis Bell. 



Presented February 8, 1911. Received February 8, 1911. 



The absorption of ultra-violet radiations by various media has been 

 the subject of numerous investigations which need not here be re- 

 counted. So far as modern optical glasses are concerned our knowl- 

 edge rests chiefly on the researches of Kriiss,^ who investigated a 

 series of ten typical Jena glasses by means of a fluorescent-screen 

 polarizing photometer. His results extended to w\ 1. 309 fifi and 

 included thicknesses of 1, 10, and 100 mm. of glasses ranging from a 

 very light boro-silicate crown of index tid = 1.51 to a dense silicate flint 

 having w^ = 1.67. At the lower limit of wave length there was practi- 

 cally complete absorption by all these glasses in a thickness of 1 cm. 

 and partial transmissibility for the lighter glasses in thicknesses of 1 mm. 

 The most striking thing about his results was the generally similar 

 form of the absorption curves and the very rapid increase of absorption 

 below w. 1. 350 nn to 340 /xju. For greater wave lengths than the first 

 mentioned, optical glasses, save for the heaviest flints, are moderately 

 transparent, as is found in ordinary practice. The writer, indeed, 

 many years ago, photographed the solar spectrum down to Cornu's 

 line 0, w. 1. 344 ft/A, through a large 45° prism of moderately dense 

 flint, W£> = 1.62. Zschimmer ^ has added to Kriiss' work an interesting 

 investigation of the so-called Uviol glasses and some other optical 

 glasses, with special reference to the value of the Uviol glass in trans- 

 mitting the shorter wave lengths in the ultra-violet. His spectrograms 

 show in some of the Uviol glasses transmission to as low as w. 1. 265 nn 

 in a thickness of 2 mm., and down to about w. 1. 295 fifj- in a thickness 

 of 10 mm. This shows, from the standpoint of transmissibility, a great 

 improvement over the best of the ordinary glasses, spectra through 

 which usually terminate at about w. 1. 300 m/x. Some extremely light 

 crowns are a trifle more transparent. 



1 Ztschr. f. Instrumentenkunde, 23, 197, 229. 



2 Ibid., 23, 360. 



