INTENSITY RELATIONS IN THE SPECTRUM OF HELIUM. 

 TABLE IX. Helium Comparison of Series. 



153 



rapidly as the distance increases, but afterwards more slowly. The true intensity of 

 the line X5876 on VII., VIII. is so small that X4713 could not appear on these 

 photographs unless this law were suddenly changed in this region, so that its absence 

 presents no difficulty. The Principal series, on the other hand, is not relatively 

 intensified at the end of the dark space, but is already decreasing in intensity. There 

 is evidence of an ultimate reversal at some distance from the cathode (on VII.) but 

 it is not conclusive. 



The intensity-ratio of Principal and Sharp series exhibited in the last row of the 

 table, clearly shows that the Sharp series tends to become stronger relatively to the 

 Principal series, rapidly at the end of the dark space, and afterwards very slowly. 

 These phenomena are very definite. 



It is perhaps desirable again to point out that the actual numbers in a table of this 

 kind are in no wny a representation of the energy emission in the various wave-lengths, 

 owing to the curve of sensibility of the photographic plate. Only the change from 

 one column to another has any significance in the present enquiry. 



In the discussion of Parhelium from the same point of view, we may confine 

 attention to the lines XX(>678, 5047, 5015, belonging respectively to the Diffuse, Sharp 

 and Principal series. The intensity of X6G7H is in each case reduced to 10. 



TABLE X. Parhelium. Comparison of Series. 



We may consider, in the first place, the cases of absent lines. The true intensity 

 of X6678 on I. is 6'25, and the absence of X5047 from this photograph merely indicates 



VOL. (JCXX. A. Z 



