148 



DR. T. R. MERTON AND PROF. J. W. NICHOLSON ON 



from the cathode, three entirely distinct lines of enquiry must be investigated. 

 These are 



(1) The relative intensities of the successive lines of any one series, as a function 



of cathode distance. 



(2) The relative intensities of corresponding lines of the Principal, Sharp and 



Diffuse series, either of Helium or of Parhelium, under the same circumstances. 



(3) The relative behaviour of the Helium lines (double) and of the Parhelium lines 



(single) in the case of corresponding members. 



The entire phenomena presented can be regarded as the result of a superposition of 

 these three effects, each of which is in itself of considerable interest in connection with 

 any theory of the origin of spectra. Such a general enquiry into one definite 

 spectrum, such as that of Helium, is necessarily somewhat long, but the spectrum of 

 Helium is, in many respects, so typical, and our knowledge of the origin of series is so 

 doubtful, that it is evidently desirable to push the investigation to the extreme limit 

 in this individual case. Only by the definite isolation of the three effects mentioned 

 can further progress in the elucidation of the nature of spectra apparently be made, 

 and quantitative measurements of intensity have not hitherto been sufficiently 

 sensitive to small changes, for the purpose of obtaining definite conclusions on any 

 one of these subjects. 



In the present section, we confine ourselves to a discussion of the relative behaviour 

 of successive lines corresponding to increasing term number in a Diffuse series. Two 

 such series are available on the present set of photographs - -the doublets characteristic 

 of the Diffuse series of Helium, and the single lines classed- generally as 

 Parhelium. The necessary data with regard to these lines in the case of Helium 

 being the joint effect of the two components of the doublet in each case are set 

 forth in Tables V. and VI. For the time being, we do not consider the interesting 

 question of the position, with respect to the cathode, of maximum emission of any one 

 line of such a series, but only relative intensities in the series on each photograph, 

 one particular line being arbitrarily chosen as 10 in every case. The results of this 

 computation are as follows : 



TABLE V. Diffuse Series of Helium. 



