148 
DR. T. R. MERTON AND PROF. J. W. NICHOLSON ON 
from tlie cathode, three entirely distinct lines of enquiry, must be investigated. 
These are— 
(1) The relative intensities of the successive lines of any one series, as a function 
of cathode distance. 
( 2 ) The relative intensities of corresponding lines of the Principal, Sharp and 
Diffuse series, either of Helium or of Parhelium, under the same circumstances. 
(3) The relative behaviour of the Helium lines (double) and of the Parhelium lines 
(single) in the case of corresponding members. 
The entire phenomena presented can be regarded as the result of a superposition of 
these three effects, each of which is in itself of considerable interest in connection with 
any theory of the origin of spectra. Such a general enquiry into one definite 
spectrum, such as that of Helium, is necessarily somewhat long, but the spectrum of 
Helium is, in many respects, so typical, and our knowledge of the origin of series is so 
doubtful, that it is evidently desirable to push the investigation to the extreme limit 
in this individual case. Only by the definite isolation of the three effects mentioned 
can further progress in the elucidation of the nature of spectra apparently be made, 
and quantitative measurements of intensity have not hitherto been sufficiently 
sensitive to small changes, for the purpose of obtaining definite conclusions on any 
one of these subjects. 
In the present section, we confine ourselves to a discussion of the relative behaviour 
of successive lines corresponding to increasing term number in a Diffuse series. Two 
such series are available on the present set of photographs—the doublets characteristic 
of the Diffuse series of Helium, and the single lines classed generally as 
Parhelium. The necessary data with regard to these lines—in the case of Helium 
being the joint effect of the two components of the doublet in each case—are set 
forth in Tables V. and VI. For the time being, we do not consider the interesting 
question of the position, with respect to the cathode, of maximum emission of any one 
line of such a series, but only relative intensities in the series on eacli photograph, 
one particular line being arbitrarily chosen as 10 in every case. Tlie results of this 
computation are as follows :— 
Table V.—Diffuse Series of Helium. 
Intensity 
A. 
Photo¬ 
graph 
I. at 
cathode. 
Photo¬ 
graph 
11 . at 
1 mm. 
Photo¬ 
graph 
III. at 
2 mm. 
Photo¬ 
graph 
IV. at 
3 mm. 
Photo¬ 
graph 
V. at 
4 mm. 
Photo¬ 
graph 
A^I. at 
5 mm. 
Photo¬ 
graph 
VII. at 
6 mm. 
Photo¬ 
graph 
VIII. at 
7 mm. 
5876 
10 
10 
10 
10 
10 
10 
10 
10 
4472 
18 
19 
15 
15 
16 
16 
15 
14 
4026 
3-0 
3-0 
3-4 
3-3 
30 
2-7 
2-5 
not seen 
