172 
DR. T. R. MERTON AND PROF. J. W. NICHOLSON ON 
In the qualitative result shown in fig. 3, we see that the Band spectrum is 
restricted to very narrow limits as regards the conditions of excitation. The 
quantitative results enable us to define in a similar way the range of conditions 
through which the different series are most strongly developed. 
The phenomena here are very definite. In the case of lines belonging to Principal 
series the seat of maximum emission is closer to the cathode, and falls away with 
increasing distance from this point more rapidly than in the case of lines belonging to 
associated series. The Diffuse series appear to preserve the most uiiiform intensity 
over a wide range of conditions. 
Whilst it is impossible to discuss these phenomena rationally, their importance in 
any comprehensive theory of the origin of spectra is evident. The “ dark space ” is a 
region in which the integrated effect of all the radiations is small, and the end of the 
dark space in the same way is a point at which this integrated effect suffers an abrupt 
change, but it is evident that the true “dark space” is different for different 
radiations, and there appears to be another point, which in our experiments was about 
5 mm. from the cathode, at which another change occurs in certain lines, whilst others 
do not appear to be affected ; but further investigation of this phenomenon is required. 
Turning now to the radiation, when the pressure in the discharge tube is very low, 
we find an entirely different phenomenon. Instead of 2 ^,progressive transfer of energy 
in the series, there is a selective transfer peculiar to certain lines. In particular the 
lines AX4388, 4472 and 5015 are relatively enhanced under these conditions, whilst 
X3888 is reduced. Of these lines X4388 and X4472 are especially prominent in the 
spectra of nebulae, but the simultaneous enhancement of X5015, which is not found in 
nebulae, shows that we have not isolated the conditions for reproducing the intensity 
relations found in the celestial spectrum, which would, however, be very closely 
represented by a super-position of the results at low pressure, and those found in a 
previous investigation when the tube was excited by a highly condensed discharge. 
No explanation can be offered as to the precise manner in which the excitation is 
altered at low pressures. 
As regards the behaviour of mixtures of Helium and Hydrogen the results have 
not quite the same quantitative significance, in the sense that there must be small 
differences in the pressure of the gas in different tubes. 
Taking account of this and other sources of error there are still changes which 
appear to be peculiar to the conditions obtaining in the mixed gases. In mixtures of 
Hydrogen and Helium, where the partial pressure of each gas is of the same order 
of magnitude, there is in general a transfer of energy in the spectra of both gases to 
the lines of higher term-number, in comparison with the distribution of intensity 
in the spectrum of Helium which was so pure that the Hydrogen spectrum 
could not be seen. On the, other hand, in the presence of what we have called a 
trace of Hydrogen, the Helium lines are affected in the opposite sense ; that is to say, 
tliere is a transfer of energy to the members of lower term-numl^er. It is remarkable 
