Secondary Radiation. 233 



= 45°. The other two experiments can be interpreted as 

 showing a maximum when the law of reflexion is fulfilled. 

 with the secondary radiation falling off unequally on the two 



sides of the maximum position. 



The numbers indicate that, superposed on an effect of the 

 nature of reflexion, we hnve other effects : the law of reflexion 

 being fulfilled, we get more secondary radiation the more we 

 approach the normal, which is to be expected, as then the 

 radiation coming from some depth in the plate has a smaller 

 thickness to penetrate. 



The general effect is therefore a radiation in all directions 

 from the part of the plate struck by the primary rays, with a 

 decided maximum in one direction, due probably to a sort of 

 reflexion of the impinging particles. Many points described 

 later show that the whole effect is not due to a simple scattering 

 of the primary rays, but that it consists of a true secondary 

 radiation from the plate. 



It may also be stated here that experiments with a magnetic 

 field, to be described later, show that the secondary rays we 

 are now dealing with consist of negatiyely charged particles. 

 The rays we are now dealing- with travel through several 

 centimetres of air, and also penetrate the tinfoil covering the 

 end of the ionization-tube T before they produce the effect 

 by which they are detected : there may be also a more easily 

 absorbed radiation from the plate, which has not been studied 

 in the present paper. 



Secondary Radiation not merely a Surface Effect. 



The secondary ray- are not produced merely at the surface 

 of the plate -truck by the primary rays ; it is easy to show 

 that they come from all parts of a layer of considerable depth, 

 the depth depending on the substance of which the plate 

 i- composed. 



This point was tested by using for the plate in fig. 1 a 

 single sheet of tinfoil (about -013 mm. thick) in the first 

 place, then two sheets, and so on. The secondary radiation 

 was as follows : — 



Sheets of Foil. 



Secondary Radiation 



1 



13 



2 



24 



4 



44-5 



8 



60-5 



Hi 



73-5 



32 



77 



