138 DR. T. R. MERTON AND PROF. 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 Pliicker 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 witli 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 recently! by FOWLER 

 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 

 DEWAR that a transfer of energy from longer to shorter wave-length in the Balmer 



* 'Phil. Trans.,' A, 1917, vol. 217 p. 237. 

 t ' Roy. Soc. Proc.,' A, 1918. 



