376 SCIENCE PROGRESS 



more simple cubic system. As it seemed possible that the effect 

 was due to a secondary X-radiation, it was deemed advisable to 

 use a crystal containing one of the heavier metals which give 

 much secondary radiation ; for this reason zinc blende was the 

 crystal chosen. 



Fig. 2B shows the result obtained when the beam of rays 

 traverses a crystal of zinc blende in the direction of a cubic axis 

 of symmetry, the interference pattern on the photographic plate 

 showing complete fourfold symmetry. 



The interference maxima are little elliptical spots arranged 

 in a complicated geometrical pattern ; these spots represent 

 narrow rectilinear pencils spreading from the piece of crystal 

 traversed by the rays. From a knowledge of the position of one 

 of the spots on the plate and of the distance of the plate from the 

 crystal, it is easy to find the direction of the pencil which formed 

 the spot in question ; and taking the cubic axes of the crystal as 

 axes of reference, to define the direction in terms of the angles 

 made with the axes. As the incident waves pass through the 

 crystal, they act on the atoms which they meet, a secondary 

 wavelet spreading from each atom as a wave passes over it. If 

 the incident beam contain a train of waves of wave length \, 

 then in order that there should be an interference maximum in 

 a particular direction it is necessary that the train of wavelets 

 from every atom in the crystal should be in phase in that 

 direction. 



To express this condition analytically, some assumption must 

 be made as to the arrangement of the centres from which the 

 secondary wavelets spread. 



Laue regards these centres as forming a point system which 

 has for its pattern a little cube with a point at each corner. This 

 is the most simple cubic point system possible. Take for con- 

 venience axes of reference parallel to the cubic axes and origin 

 at the centre of one of the atoms, molecules or whatever it may be 

 that represents the diffracting unit; then the neighbouring atoms 

 will be equally spaced in all three directions OX, O Y, OZ. 

 Let the incident light be parallel to the axis OZ and the distance 

 between neighbouring atoms be a. 



If we express the condition that the wavelets from the atom 

 at the origin should be in phase with those from its nearest 

 neighbours along OX, OY, OZ, we ensure that the wavelets 

 from all the atoms in the crystal are in phase. 



