168 
DR. T. R. MERTON AND PROF. J. W. NICHOLSON ON 
with a pressure of 1 mm. of dark space, and the spectrum of Hydrogen as shown 
under the same circumstances in conjunction with that of Helium on photograph (&). 
A comparison of these tables supplies the necessary basis for a determination of the 
effect of mixture on the spectrum of the lighter gas. 
The magnification m was in each case 3'270. The transfer of energy towards the 
higher term numbers is very evident, although in the later members, H^, &c., it is not 
completely established. The effect on the lighter gas is therefore the same as that on 
the heavier, when the quantities of each present in the mixture are comparable. In 
fact in a comparable mixture of the two gases there is a tendency in both cases 
towards relative diminution of the leading lines of series, and towards, in general, a 
shift of the energy of emission towards the violet. 
It is now clear that the mechanism of this effect must be wholly different from that 
operative when only a small quantity of Hydrogen is present. For in the latter case, 
the effect of a trace of the lighter gas on the spectrum of the heavier one is to 
transfer the energy emission of the latter towards the leading members of series, 
while a trace of the heavier gas transfers the energy emission of the lighter gas away 
from the leading members. 
(X.) Comparison of Different Series under Low Pressure. 
We have, in earlier sections, discussed the relative behawiour of different lines 
of the same series under various conditions, and have restricted the use of the term 
“ selective ” to the enhancement or reduction of any line relatively to other lines of 
the same series. Phenomena which involve the relative behaviour of different series, 
or corresponding members thereof, are, in many cases, of even greater significance. 
The classical example is the behaviour of A5015 —belonging to the Principal series of 
Parhelium—under low pressure, as compared with X5876, of the Diffuse series of 
Helium. It is well recognised that A5015 in particular is essentially a low pressure 
line. In the following table (Table XVHI.) the four lines, AA5876, 4472, 5015, 4388, 
are considered together (i) at a pressure corresponding to 1 mm. dark space and 
(ii) at low pressure. The intensities are taken from previous tables and reduced in 
each case to a scale on which the intensity of A5876 is 10. 
Table XVHI. 
A. 
At 1 mm. dark space. 
Low pressure. 
5876 
10 
10 
4472 
13-1 
31-8 
5015 
2-93 
12-7 
4388 
0-67 
3-30 
