191:5] 



on Great Advance in Crystallography 



695 



The interesting results of Bragg are in full accord with the 

 assumjition of the centred-face cubic space-lattice (No. 3), but not 

 with either the simple-cube or the centred-cube space-lattice (Nos. 

 1 and 2). They also account for the elliptical shape of the spots. 

 The amount of ellipticity depends on the distance of the photographic 

 plate from the crystal. When the two are very close the spots are 

 round, but they become more and more elliptical as the plate is 

 receded (compare Figs. 15 and 16). The phenomenon is due to the 

 fact that the initial rays are not strictly parallel, and the effect will 

 be clear from the next slide (Fig. 19). The vertically diverging rays 

 striking the reflecting planes of the upper part of the crystal meet 

 them at a less angle of incidence than those of the lower part, and 

 so the reflected rays converge. Horizontally diverging rays, how- 

 ever, diverge still more on reflection. Hence the section of the 

 reflected beam is an ellipse with major axis horizontal. 



It is of importance to note that the centred-face cubic space- 

 lattice is characteristic both of the arrangement of identically (same- 

 ways) orientated and environed atoms of the same element, zinc or 

 sulphur, and of the atoms of both elements regarded as equal spheres 

 in contact. In the slide already shown (Fig. 8, Plate II), of Barlow and 

 Pope's model, the spheres of sulphur are coloured yellow to distinguish 

 them from the grey-coloured spheres of zinc. If we ignore the colour, 

 and consider them 

 as similar spheres, we 

 see that they form the 

 centred-face cubic ar- 

 rangement. The hemi- 

 hedral nature of zinc 

 blende is, however, very 

 likely connected with 

 some real difference of 

 volume between the 

 atomic spheres. As the 

 spot figure is holohedral 

 it would appear to be 

 due to the space -lattices 

 of similarly placed atoms 

 of either (but in each 

 lattice only one) element, 

 rather than to the spheres of the combined system of atoms. 



This latter conclusion is further borne out by the result of the new 

 work by Laue on quartz. The photograph now shown (Fig. 20, Plate IV), 

 so kindly sent by Professor Laue, exhibits the trigonal nature of the 

 symmetry very clearly, and Professor Laue informs me that the same 

 figure is afforded by both right and left quartz, so that it does not 

 reveal the hemihedral character of quartz which has been demon- 

 strated this evening, but possesses the full holohedral symmetry of 

 Vol. XX. (No. 107) 3 a 



Fig. 19.— Explanation of Elliptical 

 Shape op Spots. 



