r.tlH] on Great Advance in Crystallography (i98 



we should not considtT the reflections as due to the <;eneral space- 

 lattice of zinc blende, l^aue considered the "molecules" of the 

 crystal to form a three-dimensional grating^ — that is, a Raumgitter — 

 and that each molecule is capable of emitting secondary vibrations 

 when struck by incident electromagnetic waves from the X-ray Ijulb ; 

 also that the molecules are arranged according to the simple cube 

 space-lattice (No. 1). The incident waves being propagated parallel 

 to one of the cube axes (edges), the wave-surfaces will be parallel to 

 the plane of the other two cube edges. He then considers the spots 

 to be interference maxima of the waves scattered by the orderly 

 arrangement of the molecules in the crystal. The equations of 

 condition were next found for interference maxima of direction 

 cosines a, /?, y, and for incident wave-length A, and from the position 

 of each spot the direction cosine of the pencil of rays which formed 

 it was calculated, assuming all the transmitted pencils to come from 

 the centre of the crystal. Thirteen spots in each quadrant were 

 investigated, and in every case Lane's equations were satisfied : hence 

 the conclusion that the spots are due to interference of secondary 

 Rontgen radiation appears to agree with the positions of the spots, 

 provided only radiations of certain definite wave-lengths are present 

 in the incident rays. 



The lecturer pointed out, in an article in ' Xature,' of November 

 14:th, rJ12, that the structure of zinc blende was probably not so 

 simple as had been assumed by Lane, and that the space-lattice with 

 a point at the centre of each side of the cube (No. 3) was the more 

 probable one, the structure being that assigned to it by Barlow and 

 Pope, as already described in this lecture. 



A satisfactory explanation has since been advanced by W. L. 

 Bragg, which does accord with this structure and with other essential 

 conditions referred to by the lecturer, which altogether avoids the 

 assumption of only a few wave-lengths, and which agrees with a 

 simple reflection of unchanged X-rays from the planes of points of the 

 general space-lattice of zinc blende. He regards the incident radia- 

 tion as composed of a series o': independent pulses, which, falling on 

 a number of atoms definitely scattered in a plane, are separately 

 reflected, each atom acting as a centre of a secondary wave, and 

 the whole building up a wave-front. The interference maximum 

 is thus due to the reflection of the incident pulses from a system 

 of parallel planes of similar atoms, that is, from one of the parallel 

 series of planes of the space- lattice. Now besides the principal planes 

 of the space-lattice, the cube planes, the points of the space-lattice 

 also lie in a considerable number of other planes, all of which 

 are possible crystal faces corresponding to rational indices. For 

 instance, the octahedral planes are very easily traced, as also those 

 of the rhombic dodecahedron. A minute fraction of the energy of a 

 pulse traversing the crystal will be reflected from each parallel 'plane 

 in succession, and the corresponding interference maximum will be 



