last observations, some of which we may not have interpreted 

 correctly ; but we mention them in order to show that the ine- 

 qualities that are found to exist between the emergence radia- 

 tions of various substances promise to be reducible to order 

 as soon as the difficulties of interpretation have been siir- 

 mounted. 



Let us now consider the cathode radiations on the front 

 sides of the plates. Of the cathode particles set free in B(J 

 and moving at first in the direction of the y rays, a certain 

 proportion, say p, is returned by what is beyond. These move 

 towards the face A. and a certain number of them succeed 

 in reaching it and emerging therefrom. In the case of the 

 other plat« the proportion returned is p' , The absorption in 

 B' A' is the same as in BA, because the weights are the 

 same. Comparing the two plates, stratum by stratum, we 

 find that the ''incidence" radiation of one plate is to the mci- 

 dence radiation of the otdier plate as p to p' . Now p and 

 p' are the well-known constants of the jS rays. 



When a stream of y rays is allowed to fall upon a plate 

 the cathode radiation which issues from the place of inci- 

 dence must be divisible into two parts. One consists of scat- 

 tered ^ particles derived from the stream of such particles 

 which was travelling with the y rays before incidence, and 

 which was formed during the previous transit of the screens 

 employed, solid, liquid, or gaseous. This part is scattered 

 to an extent which depends on the atomic weight of the plate, 

 according to the usual law of )8 particles. The other part is 

 originated in the plate itself in the manner just described, 

 and the amount of it is also regulated according to the fi ray 

 law. When, therefore, observers have measured the second- 

 ary radiation, due to y rays, and have found a law corre- 

 sponding to that for )8 rays, the reason of the correspondence 

 has been that they really were measuring the secondary radia- 

 tion due to )8 rays. Properly speaking, the secondary radia- 

 tion, produced by y rays, or, rather, from y rays, is propor- 

 tional to the density of the substance traversed (r/. Wigger's 

 table), and this is only another form of the law of absorption 

 of y rays. 



The relative importance of the two parts of the incidence 

 radiation just mentioned must depend on the circumstances 

 of the experiment. <i^ The researches of Kleeman (Phil. Mag., 

 Nov., 1907) show very well how the second part, which is 

 influenced by previous screening, modifies the effect of the 



(1) In a recent letter addressed by one of us to "Nature," too 

 great a preponderance Tras assigned to the first part under all 

 circumstances. 



