CONTEMPORARY ADVANCES IN PHYSICS 207 



Approximations had to be made in the calculation, as nearly always 

 in quantal problems; but they are supposed not to affect the rightness 

 of the main result. To quote Oppenheimer's description of this 

 result: "a beam of high-energy electrons should have a good part of 

 its energy converted into photons in a centimetre of lead; in an equal 

 distance these photons will be largely reconverted into pairs." 



Such was the result from which, in 1935, it was inferred that quantal 

 theory must be wrong because it was predicting something which 

 could not be found in Nature; and from which, in 1936 and thereafter, 

 it was concluded that quantal theory not only was correct but had 

 made a splendid triumph, in explaining the phenomena of showers! 

 It is not altogether clear why the later conclusion was not drawn at 

 the start; perhaps the reason is, that as lately as the summer of 1936 

 fine photographs of showers were still rather rare, while such pictures 

 as Figs. 5 and 7 with their examples of self-augmenting showers 

 had not as yet been made. On the other hand it would be premature 

 to say and misleading to imply that the process which the theory 

 describes is in exact and quantitative accord with the observations 

 on showers. There are at any rate good grounds for hoping that as 

 the mathematics of the theory is more fully worked out and the art 

 of the experiments refined, the agreement will grow better and better. 

 The most that seems safe to say is, that now we have a general scheme 

 for the interpretation of showers of a certain type, and a very hopeful 

 prospect that this general scheme will be converted into a detailed 

 and quantitative explanation as the mathematics of the theory on 

 the one hand, the aptness and precision of the observations on the 

 other hand are gradually improved. 



By inserting the words "of a certain type" in the foregoing sentence, 

 I leave open the possibility that showers may be classified into more 

 than one type, and all of these but one be ascribed to other processes. 

 This is no mere possibility but already almost a certainty. Certain 

 showers which include "heavy tracks" due to protons or still more 

 massive particles are ascribed to nuclear explosions provoked by cosmic 

 rays. If a shower fails to undergo the "multiplication" illustrated 

 in Figs. 5 and 7, it is taken as belonging to this other type. Exception 

 made for such cases, it is strongly plausible to say that shower par- 

 ticles and shower-producing particles are electrons; that accordingly 

 high-energy electrons exist among the cosmic rays, behaving as the 

 quantal theory says that they should ; and that consequently the other 

 particles, setting themselves apart from electrons by their penetrative 

 power and their failure to make showers, are of another sort. 



