1919] on Energy Distribution in Spectra 527 



called " transfer of energy alonii: a series/' For instance, in the 

 Balmer series of hydrogen, produced from pure hydrogen under 

 " ordinary " conditions, there is a perfectly definite intensity relation 

 .among the lines H^, H^, and so on ; but in the presence of helium 

 this is disturbed, and the ratios H^/Ha, Hy/H^, Hfi/H^, ... are 

 notably increased. In other words, more energy tends to be emitted 

 in the form of the more refrangible rays, at the expense of the less 

 refrangible. It is interesting to speculate as to how far this process 

 can be carried, for its logical outcome is a radiation from hydrogen 

 concentrated at A 3G46, the limit of the Balmer series. We have 

 not in fact examined the matter from this point of view. 



Neon produces the same effect on the hydrogen spectrum, the 

 .first recorded evidence being an experiment of Liveing and Dewar, 

 who found in 1900 that it was possible to observe more of the violet 

 members of the Balmer series when neon was present. AVe have 

 made quantitative measurements of the effect in various cases, and 



in one experiment, for instance, the neon was found to make Hjs - 



5 



9 11 



as strong, H^ -_ as strong, and H5 -^ as strong, in relation to Ha. 



But I shall not enter into further numerical detail. A particularly 

 interesting fact is that the effect of a small trace of an impurity is 

 often diametrically opposite to that of a large quantity, and causes 

 the transfer of energy to take place in the opposite direction along 

 the spectrum. There are evidently two quite different mechanisms 

 ■of interaction possible between the atoms of the two gases — a 

 problem I commend to the chemist for investigation. 



But it is not necessary to mix a gas with another gas in order to 

 produce the energy transfer. It can be achieved otherwise, as some 

 further slides I have here will suffice to show. We have made many 

 measurements of intensity, more especially in the spectrum of pure 

 helium, of the lines from a pure gas as dependent on the part of the 

 tube they arise from, and on the conditions of excitation. We shall 

 only consider one or two of the more interesting results which arise 

 from a comparison of three spectra of helium : (1) the " ordinary " 

 spectrum, or the spectrum given by the capillary of a vacuum tube 

 containing helium at about a millimetre pressure, excited by the dis- 

 charge from an induction coil without capacity or spark gap; (2) the 

 bulb spectrum, obtained by putting a small condenser and a very 

 small spark gap in parallel in the circuit; and (3) the capillary 

 spectrum, with a spark gap and a strong condensed discharge. In 

 both (2) and (3), the transfer of energy to the more refrangible 

 members of a series takes place very strongly. In the Diffuse Series, 

 the transferred energy goes in (3) mainly towards increased breadth 

 of the fine, but in (2) mainly towards enhanced central intensity — 

 .two very different effects. The Sharp and Principal Series show the 



