﻿Absorption of X- Rays and Fluorescent X-Ray Spectra. 987 



rather too summarily. Nor have I attempted to support that 

 view against that which he holds ; I was careful to state 

 that I did not wish to throw any doubt whatever on any of 

 the physical assumptions which form the basis of his theory 

 of ionization by collision. 



My criticisms support rather than undermine that theory. 

 I pointed out that, owing to a slip in his calculations, the 

 formula? which Professor Townsend employed to compare 

 the results of his observations with the predictions of his 

 theory were not an adequate expression of the assumptions 

 on which that theory is professedly based ; but that, when 

 the error is removed, the observed and predicted values 

 agree slightly better than before in the case of his own 

 observations, and very much better in the case of my 

 observations. 



Prof. Townsend asserts with quite unnecessary severity 

 that my remarks are either incorrect or unimportant. He 

 has not doubted the correctness of this criticism, and I can 

 hardly believe that he really regards it as unimportant that 

 his theory should agree with observation. 



I am, 



Yours faithfully, 



Norman R. Campbell. 



CI. The Absorption of X-Ray s and Fluorescent X-Ray 

 Spectra, By Professor C. G. Barkla, 31. A., D.Sc, and 



V. (Jollier, B.Sc, University of London, Kings College*. 



General Absorption Curves. 



IN a paper on " The Absorption of Rontgen Rays " t 

 Barkla and Sadler showed how the absorption by a 

 given element of beams of homogeneous X-radiation varies 

 with the general absorbability of the radiation employed. 

 [The general absorbability may be measured in any substance 

 whose spectral lines in X-rays are not within the range of 

 absorbability and not near it on its more absorbable side. 

 Aluminium is a convenient substance for all ranges usually 

 dealt with, or even the much wider range covered by these 

 experiments.] Curves showing the relation between the 

 absorption in an element R say and in Al were given, and 



* Communicated by the Authors. The expenses of this research 

 have been partially covered by a Government Grant through the Royal 

 Society. 



t Phil. Mag. May 1909, pp. 739-760. 



