INTENSITY RELATIONS IN THE SPECTRUM OF HELIUM. 



151 



from the cathode is increased. This phenomenon shows the Diffuse series of Helium 

 and of Parhelium in definite contrast, and is the first clear indication we have 

 obtained of a real difference of behaviour down the two series under the same sets of 

 conditions and excitation. We may in this connection again recall the anomalous 

 behaviour of \4388 in many celestial spectra. 



Sharp Series. The corresponding data, reduced to an arbitrary scale in each 

 case, relating to the Sharp series of Helium, are contained in Table VII. 



TABLE VII. Sharp Series of Helium. Intensity Ratios. 



The Sharp series of Helium evidently behaves like the Diffuse series in preserving 

 a constant intensity ratio between consecutive lines, at least for a distance of 3 mm. 

 from the cathode Taking the average ratio as almost precisely 5' 1 , we deduce, from 

 the true intensities 3'4, 6'2, of A4713 on photographs I., IV., that the corresponding 

 intensities of X4121 should be 07 and 1'2, of which the second should be just visible 

 and the first invisible. This is in accordance with the observational data in 

 Table VII. Thus the intensity ratio down the Sharp series does not appear to vary 

 with the cathode distance. 



The only members of the Sharp series of Parhelium shown on our photographs 

 are X5047 and \4437, which only occur together on II. It is not therefore possible 

 to examine the variations in their intensity ratio. 



Principal Series. In the region of the spectrum which we have examined, the 

 only Principal line of Helium is X3888, so that no conclusion can be drawn in the 

 present enquiry as regards the relative behaviour of the members of this series as 

 the cathode distance is varied. But the Principal series of Parhelium contains two 

 members in this region whose relative intensity on the various photographs is 

 indicated in Table VIII. 



TABLE VIII. Principal Series of Parhelium. 



