INTENSITY RELATIONS IN THE SPECTRUM OF HELIUM. 
13‘) 
series of Hydrogen is brought about by the admixture of Neon. The importance of 
the mutual effects of gases on the intensity distribution in their spectra is 
considerably enhanced by the fact that, in celestial spectra, the radiation from a pure 
gas is never in question, and if indeed the spectrum of any single element were mani¬ 
fest, its presence would not disprove the presence of other elements which, though not 
giving rise to perceptible radiations peculiar to themselves, might nevertheless exert 
an influence on the distribution of intensity in lines due to other elements. 
A third and most significant condition which affects the relative intensities of 
spectrum lines is the pressure of the gas from which they are produced. In Helium, 
as is well known, this is peculiarly conspicuous; the colour of the discharge, for 
example, being green at low pressures. The existence of this phenomenon has, in 
fact, been familiar for many years, and indeed was responsible at one time for the 
erroneous view that Helium was a mixture of two gases. This misconception was 
only removed by the demonstration that the effect in question was due to variations 
of the pressure of the gas in the tube, but there has since been no quantitative 
investigation of the nature of the changes which are known to occur. 
It is thus evident that there are a number of circumstances which modify the 
distribution of intensity in the spectrum of an element, and that in order to obtain 
further information it is desirable to investigate the simplest possible cases in which 
the nature of any changes introduced into the method of excitation of the spectrum 
can be followed in some detail. Such considerations have been the determining 
factor in the particular conditions which have been selected for study in the work 
described in the present communication. 
(H.) The Cathode Glow. 
A source of light in which we already have some definite information with regard 
to the electrical conditions is obviously presented by the glow around the cathode of 
a vacuum tube. The radiations obtained from this source in the case of Helium are 
of especial interest, for they include at once the “arc” lines, the spark line at “ 4686,” 
and also the band spectrum which Fowler has shown recently* to be of an unusual 
type, inasmuch as the heads of the bands are not related by the law of Deslandres 
appropriate for the usual band spectra, but by the Rydberg formula which had been 
regarded hitherto as applying exclusively to line series. The presence of the “ 4686 ” 
line in the same source is also interesting, as the appearance of a characteristic 
“ spark ” line in company with a band spectrum is perhaps somewhat surprising, 
although the existence of such a phenomenon shows clearly that, though the 
conditions necessary for the production of these radiations may be different, they are 
evidently at the same time not incompatible. 
* ‘Roy. Soc. Proc.,’ A, vol. 91, p. 208, 1915. 
X 2 
