SOME coxrnMroK.iRV .//>r./.vr/:.<r /.v rnvsics^ni 287 



Ih' prost'n table. Tin- impliiatioii is, that iIr- i-xpi-rinifnliTs should 

 pnu-ftil to verify the pre<lietioiis liased upon the idea, cjiiite as if it 

 were aiieptahle, while remeinherinji always that it is ahsiiril. If the 

 siircesses continue to nioinit up, the absurdity may lie contideniK' 

 exjH.'Cte<i to fade uradu.iIK- out of the piililic mind. Sucii was the 

 destiny of this heuristic idea. 



I will now descrilfe some of these wonderfulK- simple [ihenomena - 

 wDiulerfulK- simple indeed, for they stand out in full simplicity in 

 domains where the classical electromajjnetic theory would almost or 

 unite certainly impose a serious coni()lexit\-. If Planck's inference 

 from the character of the radiation within a cavity had been deferred 

 for another fifteen years, one or more of these phenomena w'ould 

 assuredK' ha\e been discovered independent h-. What would have 

 happeneil in that case, what course the e\(»lntion of theoretical physics 

 would have followed, it is interesting to conjecture. 



The photoelectric effect is the outflowing of electrons from a metal, 

 occurring when and because the metal is illuininated. It was dis- 

 covered by Hertz in 1S89, but several years elapsed before it was 

 known to be an efflux of electrons, and several more before the electrons 

 were pro\ed to come forth with speeds which vary from one electron 

 to another, upwards as far as a certain detinite maximum \alue, and 

 never lieyond it. 



Here is a rather delicate point of interpretation, which it is well to 

 examine with some care; for all the contro\ersies as to continuity 

 \ersus discontinuity in Nature turn upon it, in the last analysis^. 

 What is meant, or what reasonable thing can be meant, when one 

 saNs that the speeds of all the electrons of a certain group are con- 

 tmed within a certain range, extending up to a certain limiting top- 

 most value? If one could detect each and every electron separately, 

 and separately measure its speed, the meaning would be perfectly 

 clear. For that matter, the statement would degenerate into a 

 truism. The fact is otherwise. The instruments used in work such 

 as this perceive electrons only in great multitudes. Suppose that 

 one intercepts a stream of electrons with a metal plate connected by 

 a wire to an electrometer. If a barrier is placed before the electrons 

 in the form of a retarding potential-drop, which is raised higher and 

 higher, the moment eventually comes when the current into the 

 electrometer declines. This happens because the slower electrons 

 are stopped and driven back before they reach the plate, the faster 

 ones surmount the barrier. As the potential-drop is further magni- 

 fied, the reading of the electrometer decrea.ses steadily, and at last 

 becomes inapprecial>le. Beyond a certain critical value of the retard- 



