Section III, 1919 [67] Trans. R.S.C. 



On the Extreme Ultra-violet Spectra of Magnesium and Selenium 



By Professor J. C. McLennan, F.R.S., Mr. J. F. T. Young, M.A. 

 and Mr. H. J. C. Ireton, M.A. 



(Read May Meeting, 1919.) 



Introduction 



The present work is a continuation of that carried out by two of 

 the authors with a Hilger quartz spectrograph. Type C, in the region 

 down to 2000 A.U. The present paper deals with the spark and arc 

 emission spectra of magnesium from 2100 A.U. to 1850 A.U. with a 

 Hilger Type A quartz spectrograph, and the vacuum arc spectra of 

 magnesium and selenium from 2300 A.U. to 1400 A.U. with a specially 

 constructed fluorite spectrograph. 



In the spark spectrum of magnesium, four new lines have been 

 observed, in the arc spectrum of this metal, eleven, and in the arc 

 spectrum of selenium, some thirty-one. 



Magnesium 



Experiments 



Three sources of radiation, the spark in air, the arc in air, and the 

 arc in vacuo, were used in obtaining the spectra of magnesium. The 

 spark in air was produced by the condensed discharge of a Clapp- 

 Eastham half-kilowatt transformer, giving 10,000 volts at the second- 

 ary terminals. With this apparatus a strong thick spark was easily 

 produced. 



The arc in air was produced by using rods of magnesium metal 

 in the carbon holders of an ordinary hand-fed arc lamp. The voltage 

 applied was 200 volts, and the current varied from eight to twelve 

 amperes. With such heavy currents the metal soon became hot and 

 exposures had to be intermittent to allow for cooling of the rods. 



The type of arc lamp developed for vacuum work by McLennan, 

 Aihslie and Fuller^ was employed with the fluorite spectrograph, 

 magnesium metal rods being fastened to the electrodes. With good 

 vacua it was found that a current of 4 to 5 amperes at 100 volts pro- 

 duced a brilliant and steady arc. 



1 Proc. Roy. Soc. Ser. A. Vol. 95. Mar. 15, 1919, p. 316. 



