INTENSITY RELATIONS IN THE SPECTRUM OF HELIUM. 



167 



wave-lengths found in Diffuse Helium is repeated in the corresponding series of 

 Parhelium. For X4922 is reduced from 6'5 to 5'3. X4388 is even more reduced from 

 4'5 to 3'5, and X4144 from I'l to 07. In fact each is reduced relatively to all its 

 predecessors in the series. We are in this case dealing with a phenomenon of a 

 different type to that caused by variation of pressure, and as suggested in connection 

 with Diffuse Helium, we prefer to restrict the term " energy-transfer " to cases in 

 which the change of intensity of a line is greater than, or less than, the change in all 

 its predecessors in the series. 



The effect of a large admixture of Hydrogen is again, as in Helium, directly 

 contrary to the effect of a small trace just discussed. There is an actual enhancement 

 of the members of higher term number in the series. 



There remains the necessity of verifying the fact that the Sharp series of Parhelium 

 presents no exceptional features, and of observing from a consideration of Hydrogen 

 lines emitted in the presence of Helium in these experiments they have been 

 observed in presence of Neon in an earlier communication the simultaneous 

 effect on the lighter component of the mixture. 



For consideration of the Sharp series of Parhelium we have calculated, reducing X5047 

 to intensity 10 in each case, the following intensities of the next member, X4437 : 



X4437, intensity 7'87 in the ordinary spectrum, 7'2 with a trace of Hydrogen, and 

 6 '03 with more Hydrogen. The reduction of the second member by a trace of 

 Hydrogen is again evident, though not very strongly. A slight further reduction is 

 manifest with more Hydrogen, but the changes are so small that we may conclude, 

 from these data, that Parhelium presents no contradiction to the view that the 

 reversal of energy-transfer in the Sharp series takes place at a later stage of continued 

 admixture of Hydrogen than in the Diffuse series. The present numbers appear to 

 indicate, as did those for the Sharp series of Helium, that the reversal is on the point 

 of taking place. 



The Spectrum of Hydrogen. In the following table (Table XVII.) the results are 

 given for the spectrum of pure Hydrogen, taken in the ordinary way from a capillary 



TABLE XVII. Hydrogen. 



