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V. On Intensity Relations in the Spectrum of Helium. 



By T. E. MERTON, D.Sc., Lecturer in Spectroscopy, 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 1:J7 



(II.) The cathode glow 139 



(III.) The method of measurement 141 



(IV.) Experimental H3 



(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 



(VIII.) The regions of maximum emission 155 



(IX.) The spectra of mixed gases 159 



(X.) Comparison of different series under low pressure 168 



(XL) Further relations of different series 1G9 



(XII.) 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 

 nebulse 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. 



