X-RAYS AND CRYSTALS 



By W. L. BRAGG, B.A. 



Introductory Statement 



Since Rontgen Rays were first discovered, many experiments 

 have been made to obtain with them some effect analogous to 

 the interference, diffraction and reflection of light waves but 

 till quite recently it could be said of all these experiments that 

 they gave a negative result. X-rays are scattered and absorbed 

 by bodies placed in their path but this scattering and absorption 

 have been found to depend merely on the nature of the atoms 

 of which the body consists and no evidence has hitherto been 

 forthcoming of any influence due to the chemical combination 

 or physical arrangement of these atoms. Such effects as inter- 

 ference and reflection demand a wave front covering a large 

 area, in order that the arrangement of lines in a grating or the 

 plane surface of a mirror may impress its nature on the wave. 

 It has seemed that an X-ray represents energy limited to so 

 small a volume as to be concerned merely with the nature 

 of single atoms traversed by it, so that it could not be reflected 

 by a mirror, however perfect the polish, because it had no means 

 of distinguishing between smooth and rough. 



The experiments which form the subject of this article have 

 quite altered the aspect of the problem. An effect has been 

 obtained which shows that the regular arrangement of atoms 

 in a crystal makes its impress on the rays from an X-ray bulb 

 traversing the crystal. Not only is this so but the effect can be ex- 

 plained on any wave theory whatever, with suitable assumptions 

 about the wave lengths ; it is apparently due to the interference 

 of waves of the normal type having energy spread continuously 

 over a wave front. Except for their extremely small wave- 

 length, they are in all respects like waves of light and heat. 

 If these radiations are identical with the X-rays as investigated 

 by ionisation methods, there is the same paradox with regard 

 to their " corpuscular " and " wave " nature as there is in 

 the case of ultraviolet light. The transformation of X- into 

 cathode-rays and vice versa can be observed several times over 



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