INTENSITY KELATIONS IN THE SPECTRUM OF HELIUM. 
151 
from the cathode is increased. This plienomenon 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 VIL 
Table VII.—Sharp Series of Helium. Intensity Ratios. 
A. 
Photograph I. 
Photograph II. 
Photograph III. 
Photograph IV. 
4713 
10 
10 
10 
10 
4121 
absent 
2-0 
2 ■ 2 
just seen 
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 \4713 on photographs I., IV., that the corresponding 
intensities of A4121 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 VIL 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 \5047 and \4437, which only occur together on H. It is not therefin’e 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 di-awn 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 VHI. 
Table VHI.-—Principal Series of Parhelium. 
A. 
Photograph I. 
Photograph II. 
Photograph III. 
Photograph lA^. 
Photograph V. 
5015 
10 
10 
10 
10 
10 
3965 
2-0 
1-4 
1 -S 
2-0 
just seen 
