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V. On Intensity Relations in the Spectrum of Helium. 
By T. R. Merton, D.Sc., Lecturer in Spect^'oscopy, University of London, King’s 
College, and J. W. Nicholson, F.R.S., Professor of Mathematics in the 
University of London. 
Received April 18,—Read May 9, 1918. 
Contents. 
Section Page 
(I.) Introductory.I 37 
(II.) The cathode glow.. , I 39 
(III.) The method of measurement. 141 
(IV.) Experimental.I 43 
(V.) The Helium spectrum as a function of cathode distance.144 
(VI.) Comparison of Principal, Sharp and Diffuse series.152 
(VII.) Comparison of Helium and Parhelium.154 
(VHI.) The regions of maximum emission. 155 
(IX.) The spectra of mixed gases.I 59 
(X.) Comparison of different series under low pressure.168 
(XL) Further relations of different series.169 
(XH.) Discussion and Summary. 171 
(I.) Introductory. 
The two most fundamental characteristics of a spectrum line are its wave-length and 
its intensity, and it is very remarkable that, at the present time, while the former 
can often be expressed with an accuracy of one part in half a million, the tabulated 
intensity may frequently be affected by an error even greater than a thousand per 
cent. Yet for the elucidation of the main problems of astrophysics the relative 
intensities of spectrum lines may assume an importance scarcely inferior to that of 
a precise knowledge of their wave-lengths. Although data of the latter kind afford 
precise evidence of the presence of certain elements, and of the motions of stars and 
nebulae in the line of sight, it is to the distribution of energy in the spectrum and 
to the reproduction of specified conditions in the laboratory that we must look for 
a further knowledge of the physical and more especially the electrical conditions 
obtaining in celestial bodies. 
The changes which occur in spectra under varying conditions of excitation are' 
often of a very conspicuous character, and the study of “ spark ” or enhanced lines 
VOL. CCXX.-A 575 . ^ X [Published December 23, 1919. 
