1 . C5 



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



In the case of a cylinder (r = e) whose surface has been 

 at up to £ = and afterwards at 1 = 1, we have from (83) 

 with n = 



f^M#) W^ ^ v (90) 

 Jo A(^Fc) i> 



of which only the real part is to be retained. This applies 

 to the region outside the cylinder. 



It may be observed that when t is negative (87) must 

 vanish for positive z and (90) for r>c. 



July 19, Terling Place, Witham, Essex. 



XXXIX. The Spectra of the Fluorescent Rontgen Radiations. 

 By Charles G-. Barkla, M.A., D.Sc, Wheatstone Pro- 

 fessor of Physics, University of London, King's College*. 



THOUGH the properties of the fluorescent X-radiations, 

 emitted by various substances during exposure to 

 X-rays, have been dealt with in a number of papers and 

 notes by the writer f, the accounts have been neither com- 

 plete nor connected. The purpose of the present paper is to 

 present the subject as it stands at the time of writing, with- 

 out entering into details of experiments. This seems almost 

 a necessity in indicating lines of future research, and in 

 showing the true simplicity of what appear at first sight to 

 be very complex phenomena. 



When substances are exposed to Rontgen radiation they 

 emit X-radiations of two distinct types. One of these radia- 

 tions, like the primary in penetrating power and general 

 properties, has been termed the scattered radiation, being 

 analogous to light scattered from small particles. The other, 

 a completely transformed X^radiatiop has been termed the 

 characteristic secondary X-radiation, the homogeneous secon- 

 dary X-radiation, and latterly the fluorescent X-radiation, 

 on account of its similarity to fluorescent light. 



The scattered radiation has been fully dealt with by the 

 writer %, but it is necessary here to note its characteristics in 



* Communicated by the Author. The expenses of these researches 

 have been partially covered by a Government Grant through the Koyal 

 Society. 



t Barkla, Phil. Mag. June 1906, Feb. 1908, Oct. 1910; Proc. Camb. 

 Phil. Soc, May 1909'; Jahrbuch der Radioaktivitcit, iii. pp. 246-324. 

 Barkla and Sadler, Phil. Mag. Sept. 1907, Oct. 1908, May 1909. Barkla 

 and Avres, Phil, Mag. Feb. 1911. 



% Phil. Mag. June 1903, May 1904, Feb. 1908, Feb. 1911 ; Phil. Trans. 

 A, vol. cciv. 1905, pp. 467-479 ; Proc. Rov. Soc. A. vol. lxxvii. 1906, 

 j>p, 247-255, 



