250 
DR. T. R. MERTON AND PROF. J. W. NICHOLSON ON 
By “ordinary ” Helium spectrum is meant the spectrum given by the capillary of a 
vacuum tube containing Helium at a pressure of about one millimetre, and excited by 
the discharge from an induction coil without capacity or a spark-gap. The bulb 
discharge was obtained by putting a small condenser in parallel with a very small 
spark-gap in the circuit. The intensity of the condensed discharge was not 
sufficiently great to give rise to any appreciable broadening of the lines. This 
observation was made by means of a Fabry and Perot interferometer. It must be 
pointed out that no attempt has been made to compare the intrinsic brightness of the 
lines under different conditions. The intensities given refer to an arbitrary unit, and 
the measurements refer to the relative intensities on the photograph. It is, therefore, 
the ratios of the intensities under different conditions , for the carious lines , and not 
the intensities themselves from which conclusions may be drawn. 
A. 
Helium (ordinary). 
Helium (bulb). 
Ratio. 
K 
W H - 
Photo¬ 
graphic 
intensity. 
A. 
w H - 
Photo¬ 
graphic 
intensity. 
7065 
3-95 
9-5 
1-104 
12-7 
2-0 
0 • 232 
1-71 
0-135 
6678 
4-1 
17-2 
2-074 
118-6 
12-5 
1 • 507 
32-1 
0-271 
5876 
4-5 
18-8 
2 • 489 
308 
17-4 
2 • 303 
201 
0-652 
5047 
5-0 
4-0 
0-589 
3-88 
— 
0 
— 
— 
5015 
5-0 
12-5 
1 • 838 
68-9 
8-9 
1 • 309 
20-4 
0-296 
4922 
5 • 15 
9-2 
1 ■ 394 
24-8 
8-9 
1-348 
22-3 
0-900 
4713 . 
5-45 
12-8 
2-052 
113 
10-4 
1 • 667 
46-5 
0-412 
4472 
5-9 
14-8 
2-568 
370 
17-8 
3-089 
1227 
3-32 
4438 
6‘0 
3-0 
0-530 
3 • 39 
6-4 
1-130 
13-5 
3 • 98 
4388 
6-13 
6-8 
1-226 
16-8 
7-7 
1 • 388 
24-4 
1-46 
4144 
6-9 
9 • 9 
j-i ±j 
0-447 
2-80 
3-4 
0-690 
4-90 
175 
4121 
6-95 
5 • 0 
1-022 
10-5 
4-8 
0-981 
9-57 
0-911 
4026 
7-4 
6-0 
1 • 306 
20-2 
7-9 
1-720 
52-5 
2-60 
3965 
7-7 
3-6 
0-815 
6-53 
3-6 
0-815 
6-53 
1-00 
3888 
8-0 
8-7 
2-047 
111-4 
6-0 
1-412 
25-8 
0-232 
The ratio in the last column is the intensity in the bulb divided by that with the 
ordinary discharge, and the series of values is very remarkable. Certain lines are, in 
the bulb, centrally enhanced to a great degree at the expense of others, and since 
experiments with the interferometer show that the widths of the lines are not sensibly 
different under these conditions, the results cannot be explained on the assumption 
that they are broader in one case and, therefore, for a given energy-content, less 
intense centrally. There is an actual preferential emission of energy which is 
essentially different in the two sources. 
It is instructive to examine these ratios in the light of the known series relations 
between the lines. The Diffuse series of Helium contains the lines AA5876, 4471, 
4026, whose “ term numbers” are m = 3, 4, 5. In the bulb, the first is reduced to 
f its former central intensity, the second is increased in the ratio 3^, and the third 
