162 
DR. T. R. MERTON AND PROF. J. W. NICHOLSON ON 
particular pressure we have adopted, its relative intensity is more than doubled. 
The importance of this result, from the point of view of the conditions of pressure 
occurring in nebulfe, is sufl&ciently evident, for the behaviour of this line in nebulae as 
compared with X5876 is in the same sense. It is not unlikely that further reduction 
of the pressure may carry the process further, and this question forms an important 
subject for future investigation. 
Table XIV.—-Diffuse Series and Sharp Series (Helium). 
X. 
Reduced photographic intensities. 
{a) Low pressure. 
{b) Ordinary. 
(c) Trace of hydrogen. 
(d) With hydrogen. 
5876 
10 
10 
10 
10 
4471 
31-1 
13-1 
11-2 
19-5 
4026 
5-04 
3-69 
1-63 
3-69 
Ratio 
0-162 
0-189 
0-146 
0-190 
7065 
absent 
0-58 
0-73 
0-47 
4713 
10 
10 
10 
10 
4121 
absent 
2-4 
2-12 
1-88 
Ratio 
- 
4-16 
2-90 
4-00 
The relative intensity of A4026 with respect to Dg, on the other hand, is not altered 
so appreciably. With respect to \447l this line is much reduced. It is evident that 
the phenomenon is not correctly described as a transfer of energy to the members of 
higher term number in the series, and therefore that the effect of low pressure cannot 
be classed with the phenomena of enhancement of X4472 which we recorded in a 
previous communication. For in these cases, the line X4026 also participated in the 
effect to a much greater extent, and even relatively to X4471. 
Our conclusion must be that reduction of pressure in the tube can enhance the line 
X4472 to a great degree, but at the same time leaves other members of the series with 
nearly the same relative intensities. In the case of the Sharp series, the discussion is 
made rather more difficult by virtue of the disappearance of X7065 and X4121 from our 
plate at low pressure. But we can calculate the limiting intensities they can have. If 
their “true” intensity was unity in this case, while X4713 had its true intensity 6'44, 
they would be visible. On a scale of intensity 10 for X4713, they become visible if 
their intensity exceeds the value 10/6'44 or 1'55. We may accordingly assume that 
it is less. 
Comparing this investigation with the fact of the existence of an intensity 2'4 on 
this scale, for the line X4121 in the “ordinary” spectrum, as in the table, it is evident 
that the phenomenon found in the Diffuse series is present here also, and to the same 
degree. In the case of nebulae, the Sharp series of Helium is always very weak, but 
