I! 

 P 



Ultra-violet Spectra of Magnesium and Selenium. 451 



the lines below 2852 A.U., especially the one at 2026 A IT., 

 could be observed visually with ease by means of a fluorescent 

 eyepiece. 



The arc in air was obtained by putting rods of magnesium 

 metal in the carbon-holders of an ordinary hand-feed rect- 

 angular arc-lamp. The potential fall used was that of the 

 mains, 110 volts, and the current varied from four to six 

 amperes. For the arc in vacuo a quartz lamp of the type 

 developed by McLennan and Henderson * was used. The 

 side tubes were supplied with magnesium rod electrodes, and 

 the arc was started by bombarding the magnesium vapour 

 with electrons from the auxiliary incandescent tungsten 

 cathode. The vapour condensed on the walls of the tube 

 near the arc, but the lamp carried an additional side tribe 

 provided with a crystal quartz window and through it the 

 light passed into the spectrograph. 



With 220 volts across the magnesium terminals and a 

 current of from 8 to 10 amperes, it was found that a brilliant 

 arc could be maintained for an hour or two without the con- 

 tinued use of the Wehnelt cathode. The latter was therefore 

 always cut out of the circuit as soon as the arc struck. 



In taking photographs of the different spectra Schumann 

 plates, made by the Adam Hilger Co., were used. When 

 suitable precautions were taken to avoid fogging these gave 

 spectrograms with sharp lines and clear definition over the 

 whole range in the ultraviolet covered by the optical train. 



Some of the results obtained are reproduced in fig. 1 (PL XI.) 

 In the illustration, the upper spectrum is that of the spark 

 between zinc terminals in air. The next is that of the 

 magnesium spark in air ; and the third and fourth spectro- 

 grams are respectively those of the arc in air and the arc in 

 vacuo. In the spectrogram of the spark in air, the line 

 A, = 2852 A.U., of which the frequency is given by v = (l*5, S) 

 — (2, P), sometimes appeared reversed. In the arc in air 

 this line showed a broad though faint reversal, and the line 

 .U. was always strongly reversed. The spectrum 

 of the arc in vacuo was readily obtained without any reversals 

 showing on the plates. The line \ = 4571 A.U. was never 

 observed to exhibit reversal either in the arc or the spark 

 spectra. 



In the absorption spectrum of non-luminous magnesium 

 vapour too, no absorption was ever observed at \ = 4571 A.U. 

 This result is rather interesting since it will be recalled that 

 with the non-luminous vapours of mercury, zinc, and cadmium, 



* McLennan and Henderson, Proc. Roy. Soc. A. vol. xci. p. 485 

 (1915). 



I 



