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 A3965 on this scale, 

 on the supposition that the ratio 1'5 of IV. is preserved on V., becomes I'O 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 X3965 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 which 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 hand, 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, Sharp 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 AX5876, 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 



