394 Prof. W. H. Bragg on the Consequences of 



a stream of /3 rays falls upon a lead plate far more are turned 

 back than when the plate is made of aluminium. 



Suppose that (3 rays fall on an aluminium plate, as in the 

 figure. Suppose one of the rays to be scattered by some 

 atom in the plate at P. The chances of deflexion in various 



Fig. 1. 



directions nre represented by the radii of the deflexion oval, 

 which is roughly drawn as a dotted line. The chances of 

 emergence have now to be taken into account ; the deflected 

 ray has less chance of getting out the more parallel is its line 

 of movement to the surface. Each radius of the oval must 

 be multiplied by a factor approaching the form e~ dsece , where 

 d is the depth of the atom below the surface and 6 is the 

 inclination of the radius to the surface normal. The ends of 

 the radii thus obtained lie on a new surface which is similar 

 to McOlelland's ; its section is indicated by the firm curved 

 line in the figure. It is in the right spnse asymmetrical with 

 respect to the normal ; and the asymmetry is greater for 

 light atoms than for heavy, because the lighter the atom the 

 more eccentric is the oval. Thus McCleiland's results are 

 explained without the necessity of introducing the two hypo- 

 theses of specular reflexion and true secondary radiation with 

 all the complexities they bring in their train. 



Yet there is one way in which a sort of secondary /3 radiation 

 might occur. Can an electron in flight so collide with another 

 as to give it a large share of its energy, so that one /3 ray is 

 replaced by two of much less penetrating power? There 

 is no obligation to think so at present ; but the case is worth 

 considering, for it simplifies matters very much if we can 

 conclude that no such obligation is likely to arise. There is 

 however, not much to guide us. We may to some extent 

 argue from the behaviour of other entities. An a particle 



