310 Prof. W. L. Bragg and Messrs. James and Bosanquet : 



against sin 0, the points lie on a smooth curve, showing that 

 the intensity is a function of 6 and does not otherwise depend 

 on the indices of the face or the order of the reflexion. 



Theoretical expressions for the intensity, of the reflected 

 beam have been deduced by Darwin * and Compton f . Any 

 theoretical formula for the intensity of reflexion or diffrac- 

 tion by a crystal must contain a factor whose value depends 

 upon the number and arrangement of the electrons in the 

 atoms. In the case of reflexion, the intensity falls off more 

 quickly with increase of glancing-angle than the theoretical 

 formula would indicate, and this falling off must be ac- 

 counted for by the factor referred to above. In his paper, 

 Compton makes use of the relative measurements of intensity 

 made by W. H. Bragg, and assuming the theoretical formula 

 for the intensity of reflexion to be correct, he obtains the 

 relative value of the factor at different angles, and thence 

 deduces certain possible arrangements for the electrons in 

 the atom. 



A very interesting paper by Debye and Scherrer ± deals 

 with the same question from a slightly different standpoint. 

 The relative intensities of the diffraction haloes obtained by 

 Debye's powder method are measured photometrically. 

 The authors come to the same conclusion as had been 

 drawn from the reflexion measurements §, that the falling- 

 off in the intensity with increasing angle of scattering must 

 be partly ascribed to the variation in the factor we are 

 considering. 



The experiments described in the present paper have been 

 made with the object of extending the measurements of 

 intensity over a larger range of glancing-angles. Further, 

 a direct comparison has been made between the energy of 

 an incident homogeneous beam, and its reflexion by the 

 crystal. The results so obtained have been compared with 

 those given by the theoretical formula ; and it will be shown 

 that there is strong evidence that the formula is accurate. 

 From the observations it is possible to calculate not only the 

 relative values of the factor, which depends on the arrange- 

 ment of the electrons in the atom, but also its absolute value 

 over a range of angles, so that a direct comparison may be 

 made between the observed value and the value calculated 

 for various models of the atomic structure. 



* C. G. Darwin, Phil. Mag. vol. xxvii. pp. 315 & 675 (Feb. and April 

 1914). 



t A. H. Compton, Phvs. Kev. ix. p. 1 (Jan. 1917). 



% Phys. Zeitschr. xix. pp. 474-483 (1918). 



§ Cp. W.H. Bragg-, Trans. Roy. Soc. A, ccxv. pp. 253-274 (1915). 



