BELL. — OPACITY OF GLASSES FOR THE ULTRA-VIOLET. 673 



by uniform methods of treatment, get results which have a comparative 

 value great enough to decide most of the important questions which 

 come up for consideration in studying ultra-violet absorptions. 



Having a quartz lamp and spectrograph set up for another research, 

 it seemed desirable to the writer to take advantage of this apparatus 

 for the preliminary investigation of some of the glasses commonly 

 used in the practice of American opticians, since the data already re- 

 ferred to apply chiefly to glasses exploited mainly upon the Continent. 

 The apparatus used was the concave-grating spectrograph of the 

 Rogers Laboratory of Physics, kindly placed at the writer's disposal by 

 the Director. It is fitted with a Rowland concave grating of approxi- 

 mately 1.75 meters radius of curvature, mounted in the ordinary 

 Rowland manner. The grating itself has a ruled surface 24 X 50 mm. 

 ruled with about 14,438 lines per inch. The plateholder takes plates 

 up to 3 X 13 cm., and the whole instrument is set up in a light-tight 

 room with the slit and the source outside. The lamp employed was 

 one of the ordinary quartz mercury lamps of the French Cooper- 

 Hewitt Co., operating at about 200 volts. In these investigations it 

 was used at a normal current of 4 amperes. The lamp was set up 

 about 90 cm. from the slit, and the tube was focussed upon it by a 

 quartz lens of about 20 cm. focal length. An ammeter was kept in 

 circuit with the lamp so that the current could be adjusted to a uni- 

 form value by a rheostat. The path of the rays from lamp to plate- 

 holder was thus slightly less than 4.5 meters. Inasmuch as only the 

 violet and ultra-violet portion of the spectrum was thus investigated, 

 ordinary photographic plates (Seed's No. 27) were used. These were de- 

 veloped, each set of exposures together, in a large tray, with 5 per cent 

 rodinal for 5 minutes, and fixed together in stock hypo-solution. 

 Except for special purposes of comparison in which other exposures 

 are stated, the plates were exposed for a uniform period of 5 minutes 

 and developed as soon as each particular set had been exposed, the 

 spectrograph room being fitted with facilities for doing this. The 

 printing was likewise done in groups to secure uniformity. 



The glasses investigated included a few optical glasses which seemed 

 to be of interest, and a group of colored glasses of the kinds frequently 

 used for protective spectacles in this country. In working close to 

 the quartz lamp, which was so situated that one had to work at times 

 within half a meter, the operator's eyes were protected by spectacles 

 composed of two of the glasses referred to later, but such protection 

 seems to be quite unnecessary in ordinary working around the labora- 

 tory with this lamp. In fact the writer was convinced, during a long 

 period of experimenting, that the dangers to the eye from the quartz 



VOL. XLVI. — 43 



