Ultra-violet Spectra of Magnesium and Selenium. 459 

 Table II. — Selenium Spark Lines. 



Wave-length. Intensity. 



Wave-length. 



Intensity 



X=2155A.U. 



1 



2073 „ 



3 



63 „ 



8 



38 „ 



8 



25 „ 



2 



1993 „ 



5 



A = 1960 A.U. 10 >' 



1960 A.U. 



10 



15 ., 



3 



1897 „ 



3 



93 „ 



o 



58 „ 



7 



54 „ 



7 



2. Arc Spectrum Experiments. — An investigation was also 

 made of the spectrum of selenium in the carbon arc. Solid 

 carbons were used and a small bead of selenium metal was 

 placed on the vertical carbon. The contact was then made 

 and the selenium boiled away in the arc, which was fed 

 with a current of 10 amperes from the 110 volt direct- 

 current mains. The spectrum was photo graphed with the 

 same instrument as in the experiments with the spark, and 

 tests were made with different carbons and pieces of 

 selenium. 



The fourth reproduction " d" in fig. 2 is the spectrum of 

 the carbon arc in air, and the fifth, " <?," is that of the carbon 

 and selenium arc in air. 



Four of the best plates taken were used in making the 

 measurements, and the observations showed that but five 

 lines, all occurring in the ultraviolet, were due to the 

 selenium arc. Their wave-lengths are given in Table III. 



Table III. — Selenium Arc Lines. 



Wave-length. 



Intensity 



A=2073A.U. 



8 



63 „ 



8 



38 „ 



2 



1988 „ 



1 



60 , 



10 



30 „ 



2 



3. Absorption Spectrum Experiments. — An attempt was 

 also made to see if an absorption spectrum for selenium 

 vapour could be obtained. 



The absorption spectrum of selenium vapour under various 

 conditions has been exhaustively studied by Evans and 

 Antonoff *, who found that for high vapour-pressure there 

 is continuous absorption below \ = 5800 A.U. As the 

 vapour-pressure decreased they found that aDsorption bands 

 appeared in the green, blue, and violet, and for low pressure 

 values in the ultraviolet. No bands were discovered below 

 * Evans and Antonoff, Astrophys. Jl. xxxiv. p. 277 (1911). 



