AA SECTIONAL ADDRESSES. 
of cube faces and the two ends of a diagonal. If we put zinc atoms at 
the corners of the unit cell, the sulphur atom must lie either at the centre 
of the unit cell or at the centre of the regular tetrahedron formed by four 
of the corners of the cell: only by the adoption of one of these alternatives 
do we get the four trigonal axes. The former gives the rock-salt structure 
and is distinguished by the fact that the (100) and (110) spectra decrease 
regularly in intensity from lower to higher orders, whereas in the (111) 
spectrum the even orders are relatively greater than the odd. In the latter 
FIG. 9. 
alternative the even orders of (100) are relatively greater than the odd, 
(110) spectra are normal, and the second order of the (111) is abnormally 
small. It is easy to distinguish between the two cases. The latter is 
adopted by zinc blende. Each atom has the symmetry of Class 31, to 
which the crystal belongs, there being only one atom of each in the 
unit cell. 
In this case we are successful from X-ray measurements alone in deter- 
mining the mode of arrangement of the crystal, although the crystal is 
polar and the X-rays cannot detect polarity directly. We have been able 
to determine the structure completely, and the polarity then appears. 
