INTENSITY RELATIONS IN THE SPECTRUM OF HELIUM. 
151 ) 
the band spectrum appear together. It seems possible tliat this mode of observation 
will be of use in the resolution of complex spectra into series. 
The somewhat narrow region to which the band spectrum is confined would seem 
to imply that the conditions of excitation which give rise to this spectrum fall 
between very restricted limits. 
(IX.) The Hpectra of Mixed dases. 
A considerable amount of qualitative information has already been obtained by 
various investigators, who have examined the effect, on the spectrum of a gas, 
produced by impurities, or by a definite mixture with another gas. We have already 
referred to this work in our introductory section and in a preceding communication, 
in which wm described some strictly quantitative results shown by the spectrum of 
Hydrogen when this gas is mixed with a certain amount of Neon. The effect on the 
spectrum of Neon, which may be expected to be in some sense complementary, of the 
large admixture of Hydrogen was not investigated on account of our lack of know¬ 
ledge of the seri& relations in the Neon spectrum. In order to determine in what 
sense complementary effects occur in the spectra of the two mixed gases, it is necessary 
to select two gases whose series relations are known, and the present section of this 
communication details the experimental results obtained by our method of measurement, 
with a view to the discussion given in later sections. 
In our experiments with mixed gases, we have not been able to eliminate as a source 
of error the possibilities of effects arising from small differences in pressure in the 
different tubes, but as the effects observed do not correspond to those which would 
accompany an alteration in pressure, we feel that the observed phenomena may be 
described as particular to the mixed gases. 
We have made experiments on this subject in two cases, which present strikingly 
dissimilar phenomena. In the first case, the gas consisted mainly of Helium, with 
only a very STuall admixture of Hydrogen—sufficiently small, in fact, to justify the 
statement that practically only a trace of Hydrogen was present. In the second case, 
a considerable addition of Hydrogen to the Helium was made, so that the tube 
actually contained a “ mixed gas,” in the sense that the orders of magnitude of the 
quantities present were the same. For purposes of comparison, we also examined the 
spectrum, under like conditions, of the purest Helium which could be obtained, and 
which we have already mentioned. We shall refer to this as “ pure Helium,” and to 
a similar spectrum obtained with the purest available Hydrogen as that of “pure 
Hydrogen.” 
Allied to this investigation is another on the spectrum of Helium under very low' 
pressure, and it is convenient to record and reduce the observations relating to this 
question with those relating to mixed gases, the mode of reduction being identical. 
In the subsecjiient discussion, they may also be taken together for the sake of brevity. 
