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are, moving upward. One therefore has to await, or to produce, some 

 unusual event to reveal the sense of the traversal of a path. 



One such event is portrayed in Fig. 1. It is certainly one of the 

 most deep-seated of human convictions that when tracks are seen to 

 radiate from a common point, the objects which made them must have 



:# 



Fig. 2 — Track of a positive electron which traverses a lead plate 6 mm thick, 

 and has energy amounting to 63 million electron-volts before it enters the lead. 

 (C. D. Anderson; Physical Review.) 



travelled outward and not inward, except possibly for one which may 

 have provoked the flying-asunder of the rest. Here is such a situation. 

 The radiant point was in the midst of a mass of copper wire surrounding 

 the expansion-chamber, and it is probable though not certain that the 

 event was the explosion of a copper nucleus provoked by a cosmic ray. 

 Among the radiating paths, curvatures of opposite senses occur; and 

 this practically proves that charged particles of both signs are present. 



