[ 450 ] 



L. On the Ultraviolet Spectra of Magnesium and Selenium, 

 By Professor J. C. McLennan, F.E.S., and J. F. T. 

 Young, M.A., University of Toronto *. 



[Plates XI. & XII.] 



Introduction. 



IN view of the fact that a number of the theoretical con- 

 siderations being put forward at the present time 

 regarding atomic structure are intimately related to spectral 

 series which for many of the elements lie far down in the 

 ultraviolet, it has become desirable to make the observations 

 on the spectra in that region as extensive and as complete as 

 possible. With the object of doing so a systematic study of 

 the spectra of the elements in the ultraviolet and Schumann 

 regions was recently begun by one of us. It was proposed 

 to investigate the region between 3000 A.U. and 2000 A.U. 

 with a Hilger quartz spectrograph, Type C, that between 

 2200 A.U. and 1850 A.U. with a Hilger quartz spectrograph,. 

 Type A, and that between 2000 A.U. and 1400 A.U. with a 

 specially constructed fluorite o spectrograph. The Schumann 

 region to slightly below 600 A.U. it was proposed to examine 

 with a vacuum grating spectrograph recently designed and 

 constructed for the Physical Laboratory at Toronto by the 

 Adam Hilger Co. 



The results obtained with silicon f and with cadmium J 

 have already been published elsewhere, and the present paper 

 contains an account of the work done so far by the writers 

 on the spectra of magnesium and selenium. With the 

 former element the quartz spectrograph Type C was used,. 

 and with the latter the quartz spectrograph Type A. In all 

 some fifty-eight new lines have been observed below 2600 A.U. 

 in the spectrum of magnesium, and some fourteen new ones 

 in the spectrum of selenium. 



Magnesium. 

 I. Experiments. 

 In studying the spectra of magnesium, the spark in air, 

 the arc in air, and the arc in vacuo, were used in turn. 

 The spectrum for the spark in air was obtained from the 

 condensed discharge of a Clapp-Eastham half-kilowatt trans- 

 former rated to give 10,000 volts at the secondary terminals. 

 With this arrangement the spark was quite thick and many of 



* Communicated by the Authors. 



t McLennan and Edwards, Phil. Mag-, vol. xxx. p. 482 (1915). 

 % McLennan and Edwards, Proc. Roy. Soc. Canada, vol. ix. ser. iii. 

 p. 167 (1915). 



