146 
DR. T. R. MERTON AND PROF. J. W. NICHOLSON ON 
the interpretation of the exact meaning of the intensities requires considerations 
which are not strictly relevant to the present communication. 
Table II.—Helium Bands. 
A. 
'k. 
Photograph IV. 
at 3 mm. 
Photograph Y. 
at 4 mm. 
Photograph VI. 
at 5 mm. 
Photograph VII. 
at 6 mm. 
Ji. 
h. 
h. 
h. 
4650 
0-423 
3-1 
3-5 
2-9 
2-5 
4546 
0-439 
just seen 
1-2 
1-0 
ju.st seen 
4459 
0-456 
1-2 
1-7 
just seen 
— 
4447 
0-459 
1-1 
1-4 
just seen 
— 
4427 
0-464 
2 • 2 
2-5 
1-5 
1-2 
4414 
0-468 
2-6 
2-9 
2-0 
1-8 
4399 
0-471 
2-2 
2-5 
1-7 
1-1 
4336 
0-490 
— 
1-1 
1-5 
0-9 
For a line taken twice on the same plate, but coming from different regions of the 
discharge tube, a direct comparison of its heights in the two cases is of no value as 
an indication of relative intensities, for the difference in height corresponding to any 
definite intensity-ratio in the two cases depends very much upon the density gradient 
of the wedge, whose variations along the spectrum are considerable. We must 
accordingly, as a preliminary to au}^ discussion, obtain the photographic intensities of 
the lines in all cases, according to the formula 
logu)"' {djhim), 
for these, as we have seen, are strictly comparable for the same line on the same plate 
with two different conditions or regions of excitation. The photographic intensities 
of the series lines are given in Table III. 
The results of calculation of the photographic intensities of the Helium bands, on 
the photographs which register them,, are as given in Table IV. 
In general, we may say, in connection witli these bands, that although in a quali¬ 
tative sense they are intensified or weakened together, according to the region from 
which the spectrum is photographed, this general correspondence is not strictly 
quantitative, the relative intensities of any band in two regions being dependent to a 
small extent on the wave-length. In other words, the regions of maximum emission 
of these bands, which can appear simultaneously with the series spectrum of Helium, 
are not identical. This question will not be discussed further in this communication, 
owing to the difficulty, already indicated, of interpreting the exact meaning of the 
intensity in this case. The table already sufficiently indicates the general nature of 
the phenomena presented by the band heads in this form of experiment. 
