

<?fi»« 



13/-^; i^9 9 



Fig. 7. 



The upper figure shows the general arrangement. The Avail 

 of the C3'lindrical vessel was of brass ; a Pb or an Al lining could 

 be inserted as shown. The lower figures are diagrammatic, and 

 show the currents for different arrangements of Pb and Al at 

 top, bottom, and sides. Inversion of the plates, through which 

 the 7 rays pass into the chamber, makes little difference; but 

 there is a great alteration if the material is changed on which 

 the 7 rays fall, or the emergence radiation from the top plate. 

 The base is of less importance than in fig. o ; but the sides of 

 more importance. This should clearly be so, for geometrical rea- 

 sons. 



When the conical opening was filled by a Pb stopper, the 

 currents were all reduced considerably, but retained the same 

 proportions pretty nearlj^ 



On the other hand, Avhen a small pencil of B rays was ad- 

 mitted through a hole in the centre of the top plate, a change 

 of the material of the bottom became more effective, and of the 

 sides less effective than before ; but this difference became smaller 

 Avhen thin Al sheets were so placed as to scatter the & rays on 

 their entry into the chamber. 



In conclusion, we should like to add that Wigger was the 

 first, so far as we know, to show clearly that the secondary 

 radiation of Al, on the far side of the plate, might be greater 

 than that of Pb. A comparison of the em.ergence radiations of 

 different metals was made by Dawes (Phys. Rev., xx., p. 182), 

 who showed that they did not follow the law of the incidence 

 radiations. The same effect was indicated in the experiments 

 of Eve (Phil. Mag., Dec, 1904). We have little doubt that 

 the interesting experiment of Mackenzie (Phil. Mag., July, 

 1907) is to be explained on the lines indicated in this paper. 

 In fact, it is clear that this is the case in a broad sense : but 

 it is difficult to give a complete explanation until the laws 

 are so completely worked out that they can be applied to the 

 interpretation of experiments, which are really very compli- 

 cated, although at first sight they may seem to be simple. 



