172 DE. T. R. MEETON AND PEOF. 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 uniform 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 nun. 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 a progressive transfer of energy 

 in the series, there is a selective transfer peculiar to certain lines. In particular the 

 lines XX4388, 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 nebuke, 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, 

 there is a transfer of energy to the members of lower term-number. It is remarkable 



