Short Electromagnetic Waves by a Crystal. 57 
out so strongly by the analysis is this; that the point system to 
be considered has for element of its pattern a point at each corner 
of the cube and one at the centre of each cube face. In the 
arrangement assigned to cubical zinc sulphide and similar crystals 
‘by Pope and Barlow, this point system is characteristic of both the 
arrangement of the individual atoms regarded as equal spheres, 
and of the arrangement of atoms which are in every way identical 
as regards nature, orientation, and neighbours in the pattern. The 
atoms of zinc, for instance, in the zinc blende are grouped four 
together tetrahedron-wise, and as these little tetrahedra are all 
similarly oriented and are arranged themselves in the above point 
system, atoms of zinc identical in all respects will again be arranged 
in this point system. Which of these factors it is that decides the 
form of the interference pattern might be found by experiments 
with crystals in which the point system formed by the centres of 
all the atoms differs from that formed by the centres of identical 
atoms, 
In conclusion, I wish to thank Professor Pope for his kind 
help and advice on the subject of crystal structure. 
