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XL. The .Distribution of Scattered Rontgen Radiation. By 

 Dr. Harold A. Wilson, F.R.S., Rice Institute, Houston, 

 Texas, U.S.A.* 



I EXPERIMENTS on the distribution of the Rontgen 

 J radiation scattered by elements of small atomic weight 

 have shown that on the side of the radiator on which the 

 primary rays are incident the intensity of the scattered rays 

 is given approximately by the formula I & = I 90 (l + cos 2 6) 

 which follows from the electromagnetic pulse theory of 

 Rontgen rays developed by Sir J. J. Thomson. Here 6 is 

 the angle between the scattered radiation and the incident 

 primary radiation. 



On the side of the radiator from which the primary rays 

 emerge the scattered radiation is of much greater intensity 

 than on the other side, especially in directions near to the 

 emergent primary beam. 



Herren Friedrich, Knipping, and Laue have recently 

 described experiments on the diffraction of Rontgen rays 

 by crystals f, which Laue and W. L. Bragg J have shown 

 can be explained completely by supposing that the rays are 

 reflected by the planes in the crystal which contain a regular 

 periodic distribution of atoms. 



It is well known that metals in the solid state consist 

 largely of small crystals arranged in an irregular manner. 

 When Rontgen rays are scattered by a metal plate we may 

 therefore suppose that each small crystal in the plate reflects 

 the rays just as in the experiments of Friedrich, Knipping,. 

 and Laue. The scattered radiation from a metal plate must 

 therefore be partly due to internal reflexion from an immense 

 number of small crystals orientated at random and partly to- 

 scattering by the amorphous portion of the metal. 



Each plane which reflects the rays in any particular 

 crystal will occur in all the crystals with different orientations. 

 To calculate the distribution of the scattered radiation to be- 

 expected, it is therefore only necessary to consider the way 

 in which the intensity of the radiation from a single plane 

 varies with its orientation. 



Let the angle of incidence of the rays on a plane be i and 

 the area of the plane be u. The cross-section of the reflected 

 beam is then a cos i. The electric intensity at a point in the 

 reflected ray will be proportional to the number of electrons 

 in the plane which fall inside a FresneFs zone drawn on the 



* Communicated by the Author. 



f Sitzungsberichte der Koniglich Bayerische Ahademie der JJ'isteii- 

 schaften, June 1912. 



+ Proc. Camb. Phil. Soc. vol.xvii. Part 1 (1912). 



