138 
DE. T. E. MEETON AND PEOF. J. W. NICHOLSON ON 
has already led to results of fundamental importance, but the observation of such 
phenomena depends for its success upon the magnitude of the changes involved, and 
whereas the appearance of new series of lines under appropriate conditions is often 
apparent at once, a strictly quantitative determination of the relative intensities of 
the spectrum lines is necessary for the study of the less conspicuous changes, which 
may, nevertheless, be of fundamental importance. In particular, the intensity 
changes occurring under varying conditions in lines belonging to the same or to 
mathematically related series must be a matter for serious consideration in any 
theory of radiation which involves a theoretical interpretation of the laws of 
spectra. 
In a recent investigation"^ we have made quantitative measurements of the 
intensities of the lines of Helium and Hydrogen, and it was found that under certain 
conditions of energetic excitation, the relative intensities of the lines were altered, in 
the sense that there was a transfer of energy from the lower to the higher members 
of the various series. This phenomenon was found to occur under appropriate 
circumstances in every series investigated, although the absolute magnitude of the 
change or transfer is peculiar in each case to the individual series. The principal 
difficulty encountered in any attempt to obtain an interpretation of such results lies 
in the absence of any precise knowledge of the conditions of excitation which actually 
obtain with any specified experimental arrangement. The three cases which we 
investigated in connection with Helium were the spectrum, from the capillary of a 
vacuum tube of the Pllicker form, produced by the passage of an uncondensed 
discharge from an induction coil, and alternatively by a condensed discharge with a 
spark gap in the circuit, together with the spectrum from the bulb produced with a 
condenser in parallel and with a very small spark gap ; but in each of these cases, our 
knowledge of the manner in which the atom is excited to luminosity is not sufficiently 
definite to justify any attempt to correlate theoretically the observed intensity 
changes. 
The variations in the intensity distribution among the lines of a spectrum, produced 
by the presence of impurities or by the direct admixture of other gases, constitute 
another field for research, and in this connexion a large number of entirely distinct 
effects may occur. Quite apart from the emission of band spectra by definite 
compounds or perhaps elementary molecules, and of such spectra as the water- 
vapour bands and the ammonia bands, which have been shown recentlyt by Fowlee 
to be present in the solar spectrum, there exist such effects as the reduction of 
intensity of the band spectrum of Helium, produced by the action of certain 
impurities, and the similar action of Oxygen on the secondary spectrum of Hydrogen. 
We have also confirmed, in a quantitative sense, the original observation of Liveing and 
Dewae that a transfer of energy from longer to shorter wave-length in the Balmer 
* ‘Phil. Trans.,’ A, 1917, vol. 217 p. 237. 
t ‘ Eoy. Soc. Proc.,’ A, 1918. 
