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IX. BAKERIAN LECTURE. X-rays and Crystal Structure. 



By W. H. BRAGG, D.Sc., F.R.S., Cavendish Professor of Physics in the University 



of Leeds. 



Lecture delivered March 18, MS. received April 7, 1915. 



THE method of investigating crystalline structure by the use of X-rays has already 

 been explained in papers read before this Society. It will be convenient nevertheless 

 to re-state its principle very briefly in order to introduce some further considerations 

 which I propose to lay before you. 



The statement of the principle may be made in the following way. Let a train of 

 waves of length X be passing through a medium in which are particles having the 

 power of scattering the radiation. Suppose, further, that the scattering power is not 

 distributed evenly through the medium, but that directions can be found along each 

 of which there is a periodic variation of the scattering power of the material contained 

 in strata perpendicular to the given direction, strata being, of course, taken of equal 

 thickness for comparison. Let the distance of recurrence or spacing be called d. 

 Let be the angle between the rays and the strata. Then there will be a 

 " reflection " of the radiation by the medium of n\ = 2d sin 8, where u is any integer. 



For instance, the Lippmann process of colour photography produces such a 

 distribution of scattering power in the sensitive film through the agency of stationary 

 waves.* If light is incident on the film it is strongly reflected when X = 2d sin 6 ; if 

 the light is white the film selects the appropriate wave for reflection, and this is the 

 origin of the colour manifestation. 



f*, ^y_ ^y-^ *' ' y P 



P ^ 



Fig. 1. 



The formula is readily explained by aid of the figure, which shows a set of 

 regularly spaced planes p, p, p, ., each reflecting a minute fraction of the incident 



* 'Physical Optics,' WOOD, p. 149. 

 VOL. CCXV. A 531. 2 L (Published July 13, 1915.) 



