INTENSITY RELATIONS IN THE SPECTRUM OF HELIUM. 
169 
The other three lines are all greatly enhanced with respect to A5876. Tliey were 
selected because of the astrophysical interest attaching to these lines. The greatest 
degree of enhancement occurs in A438 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 —'inust 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 il 
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. 
(XI.) 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 
among 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, and these are 
of considerable interest in connection at least witli laboratory spectra. They may all 
be studied by relating together only one individual line of each 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.). 
VOL. CCXX.-A. 
* Loc. cit., p. 259. 
2 B 
