144 



ness of the ideas which crystallographers had already held for so 

 long about the space-lattice-arrangement of the particles in the 

 crystal. 



Indeed, Von Laue showed that the problem could be attacked 



succesfully on the basis of the ordinary diffraction-theory, the 



analytical treatment being however appreciably more complicated 



because of the ^n'dimensional nature of the grating employed. 



22. For our purpose it is better, however, not to consider these 



views in detail here, but to adopt an 

 explanation of the phenomenon brought 

 forward by W. L. and W. H. Bragg, 

 which enables the questions considered 

 here to be treated in a simple geometri- 

 cal way and to avoid all calculations. It 

 differs only from Von Laue's method in 

 form, not in essence, as several authors 

 have shown. l ) 



The principal idea of it is, that 

 the phenomena observed can also be 

 described as if the radiation were reflected 

 by the consecutive parallel and equi- 

 distant molecular layers of the crystal 

 under consideration, the "reflected" 

 vibrations interfering with each other 

 according to Huyghens' principle, 

 because each particle becomes in its 



turn the centre of a secondary wave-motion spread around it spheri- 

 cally, when a pulse of the incident beams passes over it. 



Let us suppose, that the pencil of parallel R on t gen-rays L^L Z 

 (fig. I2j) contains every possible wave-length over a wide range, 

 its spectrum therefore being a continuous one. According to our 

 suppositions, each atom of a net-plane V l struck by the primary 

 radiation, will become the centre of a new wavelet, and these various 

 diffracted wavelets will touch a reflected wave-front perpendicular 

 to the parallel beam L\ L' z which emerges from the crystal. The 

 same will be true for the atoms of the consecutive net-planes V z , V 3 , 

 etc. ; but since the rays do not usually penetrate more than e. g. one 



Fig. 123. 



i) Cf. T. Terada, Proceed, Proceed. Tokyo math. phys. Soc. 7. 60. (1913); 

 G. W. Wullf, Phys. Zeits. 14. 217. (1913). 



