PROF. W. H. BRAGG ON X-RAYS AND CRYSTAL STRUCTURE. 



261 



superimposed upon the normal rapid decline. This implies an alternation in the 

 strength of the reflecting planes. 



For, suppose the lines a, a, a, in fig. 10, represent a set of reflecting planes, 

 giving the spectra A ; and suppose also the lines b to represent a set of reflecting 



a+b, 



Fig. 10. 



planes which have half the spacing of A, and therefore give spectra twice as far 

 apart as in B. 



If we combine the two sets of planes, we compound the reflections as in 0. 



Hence we learn that the chlorine atoms have so moved away from the sodium atoms 

 that their (ill) planes lie half-way between the (ill) planes of the sodium lattice, 

 for in this way is formed a set of planes of alternating strength like the a + b, b, 

 a + b, b, of the figures. 



The structure shown in fig. 7 is in accordance with these conditions. 



Let us take the case of zinc blende, the spectra of which are given in fig. 11. 



(100) 



(no) 



(111) 



Here again the relative positions of the first spectra of the (100), (110), and (ill) 

 planes are those of the face-centred lattice ; and the actual angles of reflection show 

 that one zinc atom is associated with each point on the lattice. 



Again, we may start with a zinc lattice and suppose a similar sulphur lattice to be 

 at first coincident and then to move away to its proper place. The (110) planes are 



VOL. CCXV. A. 2 M 



