THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE, 



[SIXTH SERIES.] 



% 



APRIL 1916. 



XXXII. Note on Experiments to detect Refraction of X-rays. 

 By Charles G-. Barkla, F.R.S* 



ON the simple theory of dispersion the refractive index 

 (//,) of a substance is given by an expression of the 

 form ^2 



p? = 1 + 2 7 — o 5T 5 



irm{n z — rr) 



where N is the number, per unit volume, of particles of 

 mass m, charge e, and vibration frequency n Qi and n is the 

 frequency of the incident radiation. As the frequency of 

 incident radiation changes from n, that for luminous radiation, 

 ton', for penetrating X-radiation, each "infra-red term" is 

 diminished approximately in the ratio n 2 /n' 2 , and each " ultra- 

 violet term " (including " M," " L," and « K " characteristic 

 X-radiation terms) in the ratio — n 2 /n' 2 . But if this fre- 

 quency n' be higher than that of the natural vibration of any 

 of the constituent electrons, all these terms 



N* 2 

 wm(n* — n' 2 ) 



become negative, that is, all the vibrating systems affect the 

 refractive index in the same way. 



If then the theory were applicable even in a modified 

 form to radiation of such short wave-length as that in 

 Rontgen radiation, it seemed just possible that by careful 

 experiment refraction might in certain cases be detected. 



* Communicated by the Author. 

 Phil. Mag. S. 6. Vol. 31. No. 184. April 1916. T 



