PHENOMENA RELATING TO THE SPECTRA OF HYDROGEN AND HELIUM. 259 
other elements is of importance in that it suggests that, under ordinary conditions, 
the Principal series of an element always contains the most important part of the 
radiation. This point has hitherto been obscured on account of the vagaries of the 
photographic plate and of the eye. It is interesting to compare the absolute 
intensities of members of any one series. For example, in the Diffuse series of 
Parhelium, XX6678, 4922, 4388, the intensities are 8'12, 1'39, 0'505, or roughly 
in the ratio 16 : 6 : 1 , which fall much more rapidly than the usual tabular 
intensities. 
In the Diffuse series of Helium, XX5876, 4472, 4026, the ratios are approximately 
3‘3, 2'4, 1, and in the Sharp series XX7065, 4714, 4121, they are 11 *2, 2'2, 1, so that 
the Diffuse series of Helium falls off less rapidly than the others. 
The discussion given already of the phenomenon of transfer of energy down the 
spectrum towards the violet is perhaps sufficiently complete. It is, however, 
interesting to append a table of the relative absolute intensities of the Helium lines, 
under various conditions, with that of X3888 taken arbitrarily as 100. The results are 
deduced at once by a combination of the last table with those in previous sections, 
where the relative phenomena exhibited by the same line were examined. The 
following are the final results of the calculation :— 
Absolute Intensities of Helium Lines. 
X. 
Intensity 
(ordinary discharge). 
Intensity (bulb). 
Intensity 
(condensed discharge). 
7065 
7-61 
1-03 
very small 
6678 
8-12 
2-20 
3-30 
5876 
14-7 
9-58 
14-7 
5044 
0-498 
very small 
very small 
5015 
6-22 
1-84 
3-71 
4922 
1-39 
1-25 
1-88 
4714 
1-47 
0-606 
1-76 
4472 
10-90 
36-2 
10-90 
4388 
0-505 
0-737 
1-42 
4121 
0-678 
0-618 
1-356 
4026 
4 - 51 
11-73 
4-51 
3965 
2-60 
2-60 
3-18 
3888 
100-0 
23-2 
72-9 
The extraordinary enhancement of X4472 in the bulb, and to a smaller extent, of 
X4026, is shown in a striking way in this table, together with the weakening of X5876. 
For other points of interest, it is perhaps sufficient to refer back to the discussion of 
energy-transfer already given, but this table also indicates that the transfer takes 
place to a great extent also at the expense of the Principal series. 
It is perhaps unnecessary to give the detailed working out of corresponding 
