142 



larly distributed in space, and elongated from each other by very 

 small but definite distances. Physicists a long time ago made some 

 evaluations of the order of magnitude of these inter-atomic and 

 inter-molecular dimensions, which they found to be about 10" 1 

 or 10- 9 c.M. 



Now it will be clear that an aggregation of particles of this kind 

 will behave as a continuous body towards most physical agencies, 

 because the dimensions which come into play in such physical pheno- 

 mena, are commonly of an order of magnitude incomparably greater 

 than the extremely small inter-atomic distances mentioned above. 

 Thus, if for instance a pencil of visible light-rays travel through 

 such a crystalline body, the latter will behave towards these vibra- 

 tions like an anisotropous, but continuous medium, because the 

 wave-lengths of the luminous vibrations vary from 0,00004 to 

 0,00007 c.M., this being about ten thousand times as great as the 

 mutual distance between the consecutive particles of the assemblage. 



From special phenomena observed with R on t gen-rays, suspicion 

 had arisen among physicists, that the wave-length of these vibrations 

 which seemed to have a close analogy to ordinary light -waves, should 

 be extremely small, much smaller than those of the visible light. 

 Diffraction-phenomena studied by Haga and Wind, afterwards 

 by Walter and Pohl 1 ), and theoretical speculations by Wien, 

 Stark, and 'others, had gradually led to the conviction that the 

 wave-length of Ron t gen-radiation would be of the order of 10~~ 

 or 10~~ 9 c.M. If this were true however, the wave-length would be 

 of the same order of magnitude as the inter-atomic distances supposed 

 in space-lattices of crystals, and in that case there would exist a 

 great probability that the crystalline medium would behave no 

 longer as a continuum towards Ron t gen-radiation. It might be 

 expected that the crystal would behave towards these extremely 

 short transversal waves in a way analogous to that which the 

 well-known "gratings" in optics do towards ordinary light-waves, 

 and that a ^'//^c^'ow-phenomenon would occur, the nature of 

 which would be analogous to that which would be produced when 

 visible light fell upon a grating having three dimensions instead of 

 only two. 



21. The ingenious idea that such a crystal might be used as 



1) H. Haga and C. H. Wind, Wied. Ann. der Phys. (3). 68. 884. (1899); 

 (4). 10. 305. (1903); B. Walter and R. Pohl, Ann. der Phys. (4). 28. 715. 

 (1908); (4). 29, 331. (1909). 



