SO^fE CONTEMl'ORAHY .IIH-.-IXCFS IX PIIVSICS I'll 2^9 



thf siiinc energy as the electrons which strike the first one (minus the 

 ((iiantiiy P which, howexer, is iinnieasural)l\- small anti jwrfectly 

 iu'nli>;''»'<" in comparison with the energ>' of the electrons which 

 excite or<linar>' X-rays). This fact emerged from a series of experi- 

 ments which were performeti by various people in the first decatlc 

 of this century, the results of which were generally phraseti soirte- 

 wh.il in this way, "the energy of the secondary electrons depends 

 only on the energy of the primar> electrons, not on the nature (.f the 

 material which the primary electrons strike or on that frf)m which 

 the secondary electrons issue, nor on the distance over which the 

 X-rays tra\el." I'pon these results Sir William Bragg based his 

 corpuscular thet)ry of X-rays; for (he argued) the most sensible 

 interpretation of the facts is surely this, that some of the electrons 

 striking the first target rebound with their full energy, and rel)f)und 

 again with their full energy from the second target, each of them 

 carr>ing with it from the first to the second target a positive particle 

 which neutralizes its charge over that part of its course, and so defeats 

 all the meth(xls de\ ised to recognize a flying electron. Not many 

 years later, Sir William cooperated in the slaying of his own theory, 

 by developing the best of all methods for proving that X-rays are un- 

 dulatory and measuring their wave-lengths; but it was only the im- 

 agerv' of the theory that perished, for its essence, the idea that the 

 energ>- of the first electron travels as a unit or is carried as a parcel 

 to the place where the second electron picks it up, had to be resur- 

 rected. All the mystery of the contrast between wave-theory and 

 quantimi-theory is implicit in this phenomenon, for which Sir William 

 found an inimitable simile: "It is as if one dropped a plank into the 

 sea from a height of 100 feet, and found that the spreading rip()Ie was 

 able, after tra\elling 1,000 miles and becoming infinitesimal in com- 

 parison with its original amount, to act upon a wooden ship in such a 

 way that a plank f)f that ship flew out of its place to a height of 100 

 feet." 



Among the radiations excited from a metal by electrons of a single 

 energy E, there are many of which the frequencies differ from the 

 interpreted frequency E h, being lower. Among the electrons ex- 

 pelled from a metal by radiation of a single frequency v, there are 

 many of which the energies differ from the interpreted energy-value 

 liv, being lower. These were accounted for by supposing that the 

 electrons are troubled by repeated encounters with closeK-crowded 

 atoms. If then a metal \apor or a gas were bombarded with electrons 

 or ex[X)sed to radiation, would all the excited radiation have a single 

 frequency conforming to equation (3), would all the released electrons 



