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XC. Intelligence and Miscellaneous Articles. 



THE PRODUCTION OF FLUORESCENT RONTGEN RADIATION. 



Wheatstone Laboratory, 

 King's College. 



May 7th, 1913. 



To the Editors of the Philosophical Magazine. 



GrENTLEMEN, — 



TN the April number of the Philosophical Magazine, Professor 

 Bragg has published a letter dealing with a paper of mine of the 



previous month entitled " The Production of Fluorescent Bbntgen 



Radiation ." 



In this letter two objections are made to my conclusion, that 



the bombardment theory does not explain the production of 



fluorescent X-rays : these are : — 



(1) That the primary X-ray beam used in the experiment was 

 heterogeneous, and therefore its power of producing corpuscles not 

 sufficiently known. 



(2) That Lenard's law is assumed to be rigid. 



With regard to the first objeciion, it is necessary to point out 

 a fact which is mentioned in the paper, namely, that in determining 

 the penetrating power of the heterogeneous primary beam, a thin 

 carbon sheet was used as the scattering agent. This carbon sheet, 

 which did not absorb 5 per cent, of the primary radiation, 

 would on this account scatter the softer constituents of the 

 primary beam to a greater extent than the more penetrating rays. 

 So that for this aud other reasons, the primary radiation is in 

 reality much harder than the values given in the paper would 

 suggest. But the more penetrating this primary radiation, the 

 greater is the velocity of the expelled corpuscles, and therefore the 

 stronger the case for the theory winch the paper supports. 



In the second objection, Professor Bragg states that Lenard's 

 law is not rigid, and he brings forward evidence of a selective 

 absorption of corpuscular radiation analogous to that obtained 

 in absorption and ionization experiments. But even if selective 

 absorption does take place and supposing that this selective 

 absorption is of great magnitude, it is difficult to see how this 

 affects the conclusion drawn in the paper, namely that the bom- 

 bardment theory is not in harmony with experimental facts. 



I may add that when the second paper was written, I was 

 distinctly under the impression that Lenard's law was not rigor- 

 ously obeyed, but its general acceptance led me to think that it 

 was sufficiently accurate to justify an approximate calculation, and 

 I am strengthened in this belief by the reserve with which 

 Professor Bragg publishes his preliminary results. 



Yours faithfully, 



J, Crosby Chapman, 



