Section III, 1921 [15] Trans. R.S.C. 



On the Spectra of Helium, Hydrogen and Carbon in the Extreme 



Ultraviolet 



By Professor J. C. McLennan,- F.R.S., and P. A. Pétrie, B.A. 



(Read May Meeting, 1921) 



I. Introduction 



In a paper on the extension of the spectrum beyond the Schumann 

 region by Lyman^ some 27 wave-lengths are recorded by him as 

 coming out strongly on his plates when photographing the spectrum 

 of helium. These wave-lengths covered the spectral region lying 

 between X = 599 Â.U. and X = 1247.9 Â.U. In discussing these wave- 

 lengths Hicks pointed out that those at X = 972, 992, 1026 and 1086 

 A.U. fitted the formula for the enhanced spectrum of helium and 

 drew the conclusion that they must belong to that gas. Lyman^ 

 has, however, dissented from that view and in later communications 

 has stated that in his opinion X = 992 Â.U. is attributable to an 

 unknown origin and that X = 972 Â.U. and X = 1026 Â.U. are due to 

 hydrogen. His opinion, too, is that the wave-lengths which have 

 been recorded by him at X = 1086 A.U. and 1985 A.U. have their 

 origin in hydrogen or in some other impurity, possibly nitrogen. 



In a paper by Millikan^ also on the extension of the ultraviolet 

 spectrum a comparison is made between the Avave-lengths found by 

 Lyman in studying the spectrum of helium and those found by him 

 in photographing the "hot" spark spectrum of carbon. From the 

 values given in his table of wave-lengths it Vvill be seen that some 13 

 wave-lengths in the spark spectrum of carbon have values very close 

 to those found by Lyman in the spectrum of helium. Regarding this 

 coincidence Millikan has expressed the view that the strong group 

 of wave-lengths which appeared in all of his spectra, and which also 

 appeared in Lyman's work on helium, are to be considered as having 

 their origin in the atoms of carbon. 



1 Lyman, Ast. Phys. Jl., XLIII, No. 2, p. 89, 1916; Science, XLV, p. 187, 

 Feb. 1917. 



2 Lyman, Nature, CIV, p. 314, 1919, and p. 565, 1920; Phil. Mag., Vol. 41, 

 No. 245, p. 814, 1921. 



3 Millikan, Ast. Phys. Jl., Vol. LII, No. 1, p. 47, 1920; Ast. Phys. Jl., Vol. 

 LIII, No. 2, p. 150, 1921. 



