166 
DR. T. R. MERTON AND PROF. J. W. NICHOLSON ON 
intensity is, however, more than restored by the addition of more Hydrogen. This 
behaviour is precisely similar to that shown by the Diffuse series of Helium, although 
the actual changes are of a smaller order of magnitude. 
Diffuse and Sharp Series of Parhelium .—We stated at the beginning of the last 
section that the Diffuse series of Parhelium supplied a peculiarly exhaustive test of 
the more general applicability of some of our conclusions. The main details regarding 
the intensities of the lines under the conditions in question are given in Table XVI. 
Table XVI.—Diffuse Parhelium under Various Conditions. 
A. 
(«) Low pressure. 
(h) Ordinary. 
(c) XVith trace of 
hydrogen. 
{d) Mixture. 
6678 
10 
10 
10 
10 
4922 
12-7 
6-51 
5-27 
9-66 
4388 
11-9 
4-53 
3-55 
.5-80 
4144 
absent 
1-10 
0-70 
1-51 
4009 
absent 
0-23 
absent 
0-41 
The enhancement of X4922 and X4388 relatively to X6678 is at once obvious, by 
inspection of the table, in the case of the low pressure spectrum. It is in fact even 
more remarkable than in the corresponding Helium series. Moreover, X4388 is 
enhanced relatively to X4922. The remarks which we made earlier regarding X4472 
in the nebular spectrum apply with greater force to X4388. The behaviour of these 
two lines in nebulee is thus correlated, in the light of these experiments, by the fact 
that nelmlae are in a state of extremely low pressure—and certainly much lower than 
in the present investigation, so that the relative enhancement of X4471 and X4388 
may be expected to be much greater. But the degree to which the phenomenon 
occurs, even with the present exhaustion of the tube, is sufficiently convincing. 
As in the case of the Diffuse series of Helium, this effect again cannot be described 
as a continuous transfer of energy down the series, for if this were the case, X4144 
would become visible when enhanced to a greater degree than X4388. It is actually 
invisible, and calculation shows that this fact implies that its intensity relatively to 
X6678 is not more than doubled. We must therefore repeat the former conclusion 
that the strong enhancements of particular lines at low pressure are peculiar to these 
lines, and in fact to the three lines XX4471, 4922, 4388, of the two Diffuse series. 
Under still lower pressure, X4922 may be expected to become quite subordinated to 
X4388, which is already as strong in our experiments, and in the very low conditions 
of pressure in nebulas, XX4471, 4388, should therefore be the two most prominent 
Helium lines of the Diffuse series. This is a well-known fact of observation in 
astrophysics. 
We may now take up the consideration of the effect of a trace of Hydrogen. 
Inspection of the table is almost sufficient to show that the energy-transfer to longer 
