INTENSITY RELATIONS IN THE SPECTRUM OF HELIUM. 171 



The effect of a trace of Hydrogen is also directly in contrast in the two cases. 

 In the present case it weakens the Sharp and Principal series relatively to the 

 Diffuse. 



The addition of a comparable amount of Hydrogen, however, brings the two sets 

 of series into line, for in this case also the ultimate effect is an enhancement of the 

 Sharp and Principal series in relation to the Diffuse. In the case of the Sharp series, 

 the phenomenon is again very striking. 



We find by a comparison which it is not thought necessary to reproduce in detail 

 that when corresponding series of Helium and Parhelium, typified by their first 

 members, are compared, the following conclusions may be drawn : 



The addition of Hydrogen to Helium makes only small differences in the relative 

 radiation in any corresponding pair of lines one a single line (Parhelium) and the 

 other a doublet (Helium). But low pressure gives an enormous relative strengthening 

 of the Principal series of Parhelium with respect to that of Helium. The actual 

 ratio of relative enhancement in our experiments is about 9'0. This is, of course, one 

 aspect of the well known character of X5015 as a low pressure line. 



(XII.) Discussion and Summary. 



In discussing the results obtained, it may at once be stated that the phenomena 

 which appear to be most important are those relating to the relative intensities of 

 lines at different distances from the cathode, for in this case we are able to define in a 

 general way at least some of the conditions of excitation accompanying these changes. 

 In the experiments with mixtures of Hydrogen and Helium, and at very low pressure, 

 the observed phenomena are quite as definite, but their discussion must necessarily be 

 more of a descriptive than of a rational nature, for the latter conditions give rise to 

 changes in the mode of excitation which in the present state of our knowledge seem 

 to defy any precise specification. 



With regard to the changes at different distances from the cathode, it may be 

 stated that the electric field and the average velocity of the electrons decrease with 

 the distance from the cathode, but there is no doubt that we are at every point 

 dealing with a very heterogeneous excitation, and although we may speak of the 

 average velocity, we have no information as to the distribution of velocity of the 

 electrons. Perhaps the most striking phenomenon observed relates to the difference 

 in behaviour between the series of Helium and Parhelium, for, in the former, lines 

 belonging to a series maintain a practically constant intensity ratio at every point, 

 whilst in the latter, the relative intensity of any two lines of the same series varies 

 with the distance from the cathode. In a more general summary the seat of maxi- 

 mum emission in Helium is the same for lines of the same series, and is peculiar to 

 that series, whilst in Parhelium its position is affected by the term-number of the line 

 in the series. 



2 B 2 



