400 Prof. C. G. Barkla on the Spectra of 



of ordinary penetrating power, are the most homogeneous, 

 that is contain the smallest proportion o£ scattered radiation. 

 It was found that the amount of scattered radiation necessary 

 to produce the heterogeneity observed in the radiation from 

 copper was twice that which would be given by an equal 

 mass of the light elements H to S. Even with this admixture 

 of scattered radiation the total radiation was strikingly homo- 

 geneous, as shown by fig. 1 in the paper referred to above. 

 It is necessary to absorb an amount somewhere about 95 per 

 cent, of the fluorescent radiation before the effect of the 

 scattered radiation becomes evident. 



hi the radiation emitted under exposure to an ordinary 

 primary beam by elements of atomic weight in the neigh- 

 bourhood of Ag, Sn, Sb, ... Ge, there are two homogeneous 

 radiations superposed in addition to the scattered radiation — 

 which is more intense than from an equal mass of the light 

 elements. Special precautions have thus to be taken to 

 obtain homogeneous beams from these. Owing to the wide 

 difference in penetrating power between these two principal 

 constituents and to variations in their proportion when excited 

 by a variable primary, the secondary radiation appears hete- 

 rogeneous, and much more variable in penetrating power 

 than the primary. A small change in the penetrating power 

 of the primary frequently produces very much greater 

 changes in the character of the secondary radiation. This 

 is easily understood from the relation given later between 

 the penetrating power of the primary radiation and the 

 intensity of the secondary radiation which it excites. The 

 homogeneity of the more penetrating fluorescent radiation 

 from barium is shown in fig. 1, in which the logarithm of 

 the fraction transmitted through various sheets of aluminium 

 is plotted against the number of those sheets. The initial 

 drop is due to the easily absorbed constituent; the linear 

 portion exhibits the homogeneity of the more penetrating- 

 fluorescent radiation. The homogeneity of the more absorb- 

 able constituent was observed by Barkla and Nicol by using 

 a very soft primary beam. This method will be understood 

 from what follows. 



The radiations which have been observed from the elements 

 W, Pt, Au, Pb, Bi when these are exposed to primary radia- 

 tions of ordinary penetrating power are not so homogeneous 

 as the radiations from Fe, Cu, Zn, &c. Slight variations in 

 penetrating power may also be observed when much more 

 considerable variations take place in the primary. Whether 

 there is appreciable heterogeneity in the true fluorescent 

 radiations from these elements or not has not been very 



