8 



The lack of symmetry in the distribution of scattered X- 

 rays has been shown by Professor Bragg Z-^) and assuming, as 

 seems reasonable on many grounds, that X- and y rays 

 are of the same nature, it appears from that investigation 

 that the softer radiation shows less want of symmetry when 

 falling on a given material than does the harder. 



Now although the lack of symmetry shown by the scat- 

 tered /3 rays is much greater than that found for y and X- 

 rays, even though the former are less penetrating, the gene- 

 ral nature of the effect has been shown to be much the same 

 in the case of all three, and the difference in magnitude may 

 possibly be explained by the difference in distribution of the 

 fields of the rays concerned. 



Curves similar to C, figs. iii. and iv., have been obtained 

 for Ag and paper : they show the same general characteris- 

 tics. It is remarkable, however, that the maximum value 

 of the curve C is very nearly the same for all the substances 

 tested. 



In a recent paper by McClelland ^^^ an account is given 

 of the distribution of the returned /3 radiation from plates 

 of different substances when the incident beam of radiation 

 is inclined to the plate. The results seem capable of ex- 

 planation, in view of the effects which have just been de- 

 scribed, upon a theory of scattering without the need of in- 

 troducing the idea of a true secondary radiation proceeding 

 from the atoms affected by the incident ;8 rays. 



The general effect observed by McClelland is that the 

 distribution of the returned radiation is more uniform for 

 Pb than for Al. This is to be expected in view of the nature 

 of distribution of the scattered rays from thin films of such 

 substances as Au and Al, which has been described in the 

 present paper. 



From the results shown in figs. iii. and iv. it is at once 

 seen that the effects of scattering may considerably modify 

 the results obtained in the usual form of absorption experi- 

 ment with iS rays. The shape of the ionization chamber and 

 the positions of the screen and active material relatively to 

 the chamber and to each other may produce considerable 

 modifications in the results. 



Again, in studying the absorption of )8 rays it would 

 seem necessary to deal with very thin screens as is necessary 

 in observing the effects of scattering ; for thicker screens the 

 results are likely to become considerably complicated. 



(5' Trans. Roy. See, S.A., vol. xxxii. fl908). 

 (* Prop. R.oy. Soc, Series A, vol. Ixxx. 



