152 
DR. T. R. MERTON AND PROF. J. W. NICHOLSON ON 
The initial relative enhancement of the first member, practically at the end of the 
dark space—-a definite feature of the Diffuse series of Parhelium—is shown 
prominently in this series also. As the distance from the cathode is increased further, 
this phenomenon disappears, and the second member becomes more intense, in a 
regular manner, with respect to the first. 
This process appears to be continuous, for the true intensity of \3965 on this scale, 
on the supposition that the ratio r5 of IV. is preserved on V., becomes 1‘0 on 
calculation, which is not sufficient to render it so visible as it actually is on Photo¬ 
graph V. Evidently, therefore, the increase of relative intensity of \3965 continues, 
until there is an actual relative enhancement with respect to the first photograph. 
The Diffuse and Principal series of Parhelium thus behave similarly. 
The general conclusions, with wliich all the results hitherto detailed are in 
accordance, may be stated as follows :—- 
As the cathode distance is increased, there is no definite change of relative 
intensity in the components of any Helium series, with the possible exception of a 
slight enhancement of the first member in the Diffuse series at a considerable distance 
from the cathode. 
Parhelium, on the other liand, is in striking contrast. Earlier members of its series 
are enhanced at the expense of later members at the extremity of the dark space. 
Beyond this point, the phenomenon is gradually reversed, until finally there is a 
definite enhancement of later members at the expense of those of lower term- 
number. 
This difference of behaviour of the single-line and doublet series must be of 
importance to any discussion of their origin. From a general point of view, it 
appears to imply at least that the two sets of series are not produced from the same 
atoms. 
(VI.) Comparison of Principal, Shmp and Diffuse Series. 
Superposed on the phenomena investigated above is another of considerable 
interest—-the variation in the relative intensities of corresponding members of the 
three series of Helium or of Parhelium. The conclusions already reached as to the 
uniformity of behaviour for example in the three series of Helium, render it 
unnecessary to discuss the validity of this comparison based on corresponding 
members, for the conclusions to be obtained in this section are not dependent, in 
consequence, on the particular corresponding members selected for illustration. 
In the case of Helium, we shall select the lines \A5876, 4713, 3888, as representatives 
of the three series, reducing the first to intensity 10 on a new scale for each photo¬ 
graph. The results are indicated in Table IX. 
The intensity of the Sharp series, after a definite increase, again at the end of the 
dark space, continuously decreases with reference to that of the Diffuse, at first 
