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



penetrating portion of the primary beam exhibited greater 

 polarization than the more absorbable portion. These results 

 may be explained from a consideration of the origin of primary 

 and secondary rays in the anticathode of an X-ray tube. 

 Sufficient experiments have, however, not been made to 

 permit one to say whether these results are perfectly general 

 or not. 



The distribution of the scattered radiation in a plane 

 containing the direction of propagation of the primary beam 

 agrees remarkably well with the theoretical distribution 

 within wide limits. The intensity Ig in any direction making 

 an angle 6 with the direction of propagation of the primary 

 beam is given by Jq = 1tt (1 + cos 2 0). There is, however, a 



2 



large discrepancy between the theoretical and experimental 

 values when 6 is less than 30°. From the accuracy of the 

 agreement for all other values of 6, it may be concluded 

 that this discrepancy is due to some special conditions in 

 directions near to that of propagation of the primary beam. 

 Further experiments are at present being made to determine 

 these. It is unnecessary at this stage to consider various 

 hypotheses in explanation of the experimental result. 



The total fraction of the energy of primary radiation which 

 is scattered depends on both the penetrating power of the 

 primary radiation and on the particular substance traversed 

 by the primary beam. The elements from hydrogen to 

 sulphur scatter the same amount mass for mass ; that is 

 when a given beam passes through sulphur and oxygen say, 

 the scattering is tvviceas great per atom of sulphur as per 

 atom of oxygen. From heavier elements it appears that 

 the scattering is greater mass for mass than from light 

 elements. From copper, for instance, after the fluorescent 

 radiation had been absorbed there remained twice as much 

 radiation, presumably scattered radiation, as from an equal 

 mass of the light elements. From silver, too, a radiation 

 showing practically full polarization effects, and therefore a 

 scattered radiation, was of the order of six times the intensity 

 of that from the light elements. These results may only be 

 taken as giving the order of magnitude of the relative inten- 

 sities. Further work on other elements under exposure to 

 various primary beams is needed. 



The fluorescent X-racliations emitted by elements exposed 

 to an ordinary beam of X-rays have been described in early 

 papers by the writer and Dr. Sadler as homogeneous secon- 

 dary X-radiations, or as characteristic secondary X-radiations, 



