•414 Prof. W. H. Bragg on the Consequences of 



to the axis of x. Experiment shows that the absorption curves 

 for /3 rays in aluminium screens do really possess a trace of 

 this peculiarity, for they fall slowly at first and much more 

 quickly afterwards. On the other hand, the absorption curve 

 for lead is more like an exponential curve, which is to 

 be expected since scattering is the most prominent cause of 

 absorption. 



Sir J. J. Thomson has recently published (Proc. Camb. 

 Phil. Soc. xv. part v. p. 465) a theory of the " scattering of 

 rapidly moving electrified particles/'' It seems to me to be 

 inapplicable to the actual case because it considers scattering 

 to be due to a multitude of small deflexions experienced by 

 the particle in passing by the various centres of positive and 

 negative force in the atoms, all the centres acting indepen- 

 dently of each other. Apart from the question as to whether 

 it is likely that the positives and negatives do not interfere 

 with each other's actions, the argument is limited to cases 

 where the total deflexion is so small that the particle has 

 hardly moved from its original direction when it emerges on 

 the other side of the screen. This is necessary because the 

 deflexion is taken to be the average of a number of deflexions, 

 and the reasoning tacitly assumes that all these deflexions 

 are grouped symmetrically about the original direction of 

 the particle throughout the whole of the transit of the particle 

 across the absorbing layer. The scattering of a pencil of 

 /3 rays is looked upon as a gradual opening out of the whole 

 pencil, and the calculations refer to a state in which the 

 absorbing layer is so thin that only slightly scattered rays 

 are worth considering. Actually there is no such state ; 

 however thin the plate the highly scattered rays are in a 

 certain proportion to the slightly scattered rays, which does 

 not alter as the thickness of the layer is increased, unless the 

 thickening is carried too far. From the very first large 

 deflexions must be considered. The many slight deflexions 

 which the /3 particle experiences along the comparatively 

 straight portions of its track are of no real consequence ; 

 little more than in the case of the a particle. Moreover, 

 while the plate is still fairly thin, another important effect 

 comes in 7 viz. the loss of speed ; and it is by the mutual 

 interplay of these two that the differences in the absorption 

 curves are caused. Crowther (Camb. Phil. Soc. Proc. xv. 5, 

 p. 442) shows absorption curves of aluminium and of platinum. 

 The curves show the special characteristics just discussed ; 

 but I think it is only by accident that the aluminium curve 

 fits the formula derived by Sir J. J. Thomson. The curve 

 for platinum will not fit the theory in the same way, and 



