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IX. Bakerian Lecture.— On X-rays and the Theory of Radiation. 
By C. G. Barilla, F.R.S., Professor of Natural Philosophy in the University of 
Edinburgh. 
Lecture delivered May 25, 1916,—MS. received January 3, 1917. 
INTRODUCTION. 
Of the various possible methods of approaching present day problems of electro¬ 
magnetic radiation—particularly the problem of reconciling with the older electro- 
magnetic theory those experimental facts which have led to the formulation of the 
Quantum Theory—the most profitable is probably to be found in the study of the 
phenomena of Bontgen radiation. 
Primarily this is due not to the accuracy which may be attained in measurements 
upon radiation of this type, though this is an important factor, but to the 
fundamental nature of the phenomena themselves. The result of this is shown both 
in the simplicity and in the generality of the laws observed. 
My purpose in this lecture is to call attention to some of the most significant 
experimental facts bearing on the nature of the processes resulting in the emission of 
X-rays—dwelling especially on the work I have been able to do in the last few 
years. Some of this I indicated in short notices appearing in ‘ Nature at the 
beginning of last year, but the communications were too brief, perhaps, to be fully 
intelligible. Although the war has naturally retarded the work since then, certain 
facts have been established sufficiently conclusively, and, what is important, are 
sufficiently simple in themselves, to justify a further and more complete announce¬ 
ment. And though the theory put forward will provide subjects for investigation for 
some years to come, much might be lost by waiting for more complete evidence. 
I therefore propose to give in outline the experimental basis for this theory. (A 
detailed description of the experiments themselves may be left for publication 
elsewhere.) In addition, the results of earlier work which has a direct bearing on 
recent problems of radiation will be incorporated. The significance of much of this 
work has probably been overlooked in discussions on the theory of radiation. 
The known processes resulting in the emission of X-rays will be considered under 
* “Problems of Radiation,” ‘Nature,’ February 18, 1915, and “X-ray Fluorescence and the Quantum 
Theory,” ‘Nature,’ March 4, 1915. 
vol. ccxvii.—a 557. 2 y 
[Published, August 29, 1917. 
