Emission Spectrum of the Mercury Arc. 685 



of a number o£ the elements, he concluded that it was due to 

 the presence of hydrogen. In these later measurements a 

 bolometer was used in combination with a grating. More 

 recently still H. Rubens and 0. von Baeyer* have succeeded 

 in showing that the mercury arc emits a radiation of wave- 

 length about 313 /x. They succeeded in isolating this radia- 

 tion by the method of focal separation previously used by 

 Rubens and Wood f, and in measuring its wave-length by 

 means of a Fabry and Perot interferometer of a special type 

 in combination with a Rubens microradiometer. Subsequent 

 measurements { by them on this radiation showed that it 

 consisted of two wave-lengths, the one at about 218 jjl and 

 the other in the neighbourhood of 343 /jl. 



The only noteworthy measurements by photographic 

 methods in the infra-red spectrum of the mercury arc were 

 made in 1912 by H. Lehmann§. He used the phosphoro- 

 photographic method of Bergmann ||, and found four lines 

 corresponding to those of Paschen and in addition a new 

 line at 1*46 p. 



In the present investigation a careful survey was made of 

 the infra-red spectrum of the mercury arc in the region 

 beyond 1*0 yu- with the object of confirming, if possible, the 

 existence of the lines identified by Paschen, and of seeing 

 whether the lines found by Coblentz and Geer in the neigh- 

 bourhood of 3 fi had a real existence. In this examination 

 a number of the lines noted by Paschen were identified, the 

 existence of a line near 3*0 fi was confirmed, and in addition 

 a number of new lines were observed. 



II. Apparatus. 



In this work a number of different forms of mercury arc- 

 lamps were used as sources of the radiation, but a quartz 

 mercury-lamp constructed by W. C. Heraeus was found to 

 be the most satisfactory. This lamp gave a very powerful 

 and quite concentrated arc. When in operation it was 

 driven with a direct current of from 3'0 to 32 amperes with 

 a striking potential of 110 volts. A suitable resistance was 

 of course inserted in series with the lamp. It was found 



* H. Rubens and 0. von Baeyer, Phil. Mag. xxi. pp. 689-695 (1911). 



f H. Rubens and R. W. Wood, Preuss. Akad. Wiss. Berlin, Sitz.- 

 Ber. lii. pp. 1122-1137 (1910). 



\ H. Rubens and O von Bae}-er, Preuss. Akad. TViss. Berlin, Sitz.- 

 Ber. xxx. pp. 606-667 (1911). 



§ H. Lehmann, Ann. d. Phys. xxxix. 1. pp. 76-77 (1912). 



11 Bergmann, Zeitschr. Wiss. Phot. vi. pp. 113-130 and pp. 145-169 

 (1908). 



