494 Scientific Proceedings, Royal Dublin Society. 



tlie intensity or brightness of the light. This at once follows, if we assume 

 that each expelled electron absorbs one quantum of energy, and witli this 

 store starts upon its path. Tlie number of quanta falling per second upon 

 unit area of the substance increases, indeed, with the intensity, but each 

 individual quantum remains the same in amount; and thus while the number 

 of electrons liberated from the illuminated surface increases with the intensity, 

 tlie velocity of the electrons is determined entirely by the value of n. 



The fact that -y and X rays give rise to very fast and penetrating j3 rays 

 when the former are absorbed in matter has, for some years, been known to 

 science, and we are able to state tiiat here, too, and over a much greater 

 range than that of the whole visible spectrum, tho law obtains that the 

 velocity of the expelled electron increases with the frequency. What is more, 

 we are safe in stating, from the law of quanta, that such j3 rays greatly exceed 

 in energy those expelled by light-waves ; and we are thereby enabled to 

 give an explanation of their observed much greater power of penetration. 



The mechanism by which a light, a j, or an X ray is transformed into j3 

 radiation is unknown. It is most certainly an inter-atomic process; that is, 

 it is consummated within the atom. Whatever the explanation may be, it must 

 involve a reciprocal or reversible mechanism ; for it is established that the 

 /3 ray can give rise to the y ray, the quantum of energy being handed from 

 one form of activity to the other. 



In the case of light a similar transformation explains — according to 

 Einstein, Leiiard, and Saelaud — the phenomena of fluorescence and phos- 

 pliorescence. Here the liglit-wave, imparting its energy to the electron, 

 liberates it from the atom. The electron after a longer or shorter sojourn 

 in the precincts of the parent atom is finally attracted back^a return which 

 may be accelerated by the agitation involved in a rise of temperature. The 

 disturbance attending the return of the electron to the atom sets up 

 vibrations of visible length, and, in fact, gives rise to the phenomena of 

 phosphorescence and fluorescence. These vibrations represent the restoration 

 into aethereal waves of some of the original lumiuous energy. The fact that 

 there is some loss of energy in the processes of transformation involves 

 smaller quanta in the final luminous vibrations and hence longer wave- 

 lengths. This realizes Stokes's law, wliich defines the wave-length of the 

 fluorescent light to be longer than that of the exciting liglit.^ 



In the X ray tube the electric force between catliode and anti-cathode 

 sends a stream of fast moving electrons from the one to the other, and it is 

 wliere this stream impinges on the atoms of the anti-cathode that X rays 



' Einstein, ".inn. d. Physik.," Bd. xvii, 1905. Lenard and Saeland, "Ann. d. Physik.," 

 Bd. xxviii, 1909. 



