RECENT ADVANCES IN SCIENCE 541 



In addition, a higher value for the impact voltage would give 

 the bombarding electrons power to transmit sufficient energy 

 to the orbital electrons to carry them from an inner orbit to 

 the next but one, and then would arise the possibility of the 

 electron returning to the next orbit as well as to the inner, 

 giving rise to the emission of two monochromatic radiations, a 

 view for which there is some experimental evidence referred 

 to earlier. In fact the theoretical possibilities widen out with 

 increasing impact voltages up to the ionisation limit, although 

 we still await more precise experimental proof ; and it would 

 appear that the following view is probably sound. "It is 

 possible to cause a vapour bombarded in a vacuum by elec- 

 trons to emit a radiation consisting of one wave-length and 

 one wave-length only, provided all the bombarding electrons 

 possess kinetic energy given by the quantum relation Ve = hf, 

 f being the frequency of the monochromatic radiation 

 stimulated ; moreover, it would appear that by gradually 

 increasing the speed of the bombarding electrons the vapour 

 may be caused to emit at successive stages radiation of shorter 

 and shorter wave-lengths, each particular wave-length being 

 stimulated only when the bombarding electrons have attained 

 velocities corresponding to its frequency as indicated by the 

 quantum relation." 



The natural limit of this process is, of course, when ionisa- 

 tion occurs and then all the lines of the element's spectrum 

 become possibilities, and of course among them the one of 

 shortest wave-length and greatest frequency or wave-num- 

 ber ; thus one would expect the convergence frequency to 

 appear when the ionisation potential is reached ; and it is a 

 striking fact, already mentioned, that the ionisation potentials 

 of mercury, zinc, cadmium, and magnesium (viz. 10-4, 9*4, 

 9 and 7-6 volts respectively) correspond to the convergence 

 wave-numbers of their singlet principal series 



9 = (1 "5, «)-(», £) n = 2, 3, 4... 



this convergence wave-number being (1*5, a) corresponding to 

 m = 00. Such a convergence number corresponds in fact to 

 the return of an electron from without to an innermost orbit, 

 just as the numbers corresponding to n = 2, 3, etc., correspond 

 probably to electrons jumping from the second, third, etc., 

 orbits to the inner one. 



