581 Prof. J. J. Thomson on the Emission of 



hardly be established for sequences of less than ten thousand 

 similar waves. Perhaps the only means of even roughly 

 guessing at the time of optical relaxation is by the time-lag 

 in such phenomena as fluorescence, which are connected in 

 part with free internal vibrations excited in the elements o. 

 the medium. Stated in the present form, the criterion that 

 a Rontgen aether-pulse should be regularly refracted and 

 dispersed into wave- trains, according to a process of which 

 Lord Rayleiglr's rationale has been paraphrased above, is 

 that its duration should be long compared with the time of 

 optical relaxation of the dispersing medium. In the hydro- 

 dynamic illustration the restriction does not arise, for the 

 time of molecular relaxation is far beneath the period of any 

 observable surface-waves. 



To sum up, it now seems clear that Lord Eayleigh's 

 application of the phenomena of a maintained moving source 

 gives an adequate picture of the modus operandi of the 

 dispersion of an incident aperiodic disturbance into regular 

 wave-trains by refraction, for all types of disturbance that 

 are slow compared with the period of natural molecular 

 relaxation of the refracting medium, — provided, however, 

 anomalous dispersion, which cannot be included unless a 

 ^wtfsi-frictional term is assumed in the analysis, plays a part 

 which is unimportant. But it is still held to be unlikely 

 that sethereal pulses of the type of the Rontgen rays come as 

 a rule within this limit. If this be so, white light, such as 

 can be regularly dispersed by a prism, cannot consist of 

 wholly irregular setherenl disturbance : each Fourier com- 

 ponent, comprised within say the infinitesimal range of 

 wave-length between \ and \-f-S\, must have sequences 

 of regularity in its amplitude, of duration comparable with 

 the time of optical relaxation of the dispersing medium. 



Cambridge, October 10, 1905. 



LXIV. On the Emission of Negative Corpuscles by 

 the Alkali Metals. By J. J. Thomson, M.A., F.R.S* 



IT is well known that the alkali metals when exposed to 

 light give out negative corpuscles, even when the light 

 is of very feeble intensity. Thus Elster and Geitel found 

 that the light emitted by a piece of glass rod heated to a 

 dull red heat was sufficient to make rubidium emit corpuscles. 

 It has not, however, as far as I am aware, been noticed that 



* Communicated bv the Author. 



