INTENSITY RELATIONS IN THE SPECTRUM OF HELIUiM. 1 69 



The other three lines are all greatly enhanced with respect to X5876. They were 

 selected because of the astrophysical interest attaching to these lines. The greatest 

 degree of enhancement occurs in X438 8, which would suggest the conditions obtaining 

 in nebulae, but there is an almost equivalent enhancement producing in fact an 

 actual greater intensity in X5015. In a previous section of this communication, we 

 concluded that the relative behaviour at low pressure, as regards their own individual 

 series, of XX4472 and 4388, reproduced the intensity relations found in the nebulae. 

 But the corresponding effect for X5015 shows that we have not isolated the conditions 

 which produce the nebular intensity relations in this way. Some further condition, 

 which is capable of producing a diminution of intensity in X5015 with perhaps a 

 further enhancement of XX4472 and 4388 -must be superposed. A consideration of 

 the results of a previous communication* relating to the effect of a condensed 

 discharge, suggests that the great intensity of excitation obtaining under such 

 conditions may be the necessary co-operative condition. It is at least evident that if 

 we could superpose the effects separately obtained under the two conditions very 

 low pressure and the condensed discharge a very close approximation to the actual 

 intensity relations of the Helium spectrum in nebulae would be realised. 



(XL) Further Relations of Different Series. 



In the previous section we have considered one special problem of interest to 

 astrophysicists the reproduction in the laboratory of the intensity distribution 

 amonp Helium lines as known in nebulae. It is to a great extent a problem involving 

 mainly the lines whose behaviour is in some degree exceptional. But there are in 

 addition a number of relations which may be described as more normal, arid these are 

 of considerable interest in connection at least with laboratory spectra. They may all 

 be studied by relating together only one individual line of eacli of the series the 

 first member except in one case, the Sharp series of Helium, where it is more 

 convenient to use X4713 rather than X7065. Apart from such exceptional lines as 

 XX4472, 4388, the general intensity relations of the series are sufficiently defined by 

 those of the particular representatives selected below for comparison. It is of course 

 understood that all the effects now to be discussed are superposed on the various 

 purely selective effects selective as regards one particular series in each case 

 discussed in all sections, after the experimental description, except the last, which 

 deals with a purely special problem. 



We shall take, in the first place, the available data on the general emission in the 

 three Helium series under various conditions, these series being represented by the lines 

 X5876 (Diffuse), X4713 (Sharp) and X3888 (Principal). The arbitrary intensity of the 

 first is 10 in every case in the table (Table XIX.). 



* Loc. cit., p. 259. 

 VOL. CCXX. A. 2 B 



