INTENSITY RELATIONS IN THE SPECTRUM OF HELIUM. 161 



TABLE XIII. Intensities of Helium Lines under Various Conditions. 



Helium Series- under Various Conditions of Pressure and of Purity. We may 

 begin the discussion of the phenomena contained in the last table by a consideration 

 of the Diffuse and Sharp series of Helium. The relative intensities of lines belonging 

 to different series, including the classical example of A587G and A5015 at low 

 pressures, will be considered in a later section. 



We shall for the moment confine attention to the question of energy transfer up or 

 down the series, from one line to another of the same series, produced by very low 

 pressure as distinguished from "ordinary" conditions of pressure or by admixture 

 of a large or small quantity of Hydrogen. In each photograph we reduce the 

 intensity of \5876, in discussing the case of Helium, to 10 on any necessary scale, 

 with corresponding calculations of the reduced intensities on the same scale, of A4471 

 and X4026. The results are shown, with the corresponding ones obtained in the same 

 manner for the Sharp series -represented by the lines AA7065, 4713, 4121 in 

 Table XIV. There are three members in each case, and we have accordingly 

 appended also the intensity ratio of the two other members, as they must also be 

 compared with one another as well as with the first member. In the case of the 

 Sharp series it has been more convenient to take A4713 as the standard instead of 

 A7065. 



In discussing these results we must, of course, take the ordinary spectrum of pure 

 Helium given on photograph (b), and refer the others to this as a standard. 

 Inspection of the table reveals the following main characteristics of these spectra : 

 Low pressure definitely enhances the line \4471 with respect to A5876, and at the 



VOL. coxx. A. 



2 A 



