Crystalline Structure of Antimony. 237 



(c) (110) planes. 



The planes arc again in pairs, each plane containing an 

 equal number of atoms. The spacing is 0*141: 0*859. The 

 intensities of the first three orders should have the ratio 



100 : 17 : 1. 



The observed ratio was about 



100 : 20 : 0. 



(d) (110) planes. 



These planes are parallel to the trigonal axis. They are 

 evenly spaced, and should show a normal sequence of spectra. 

 No certain third-order spectrum was found, but the glancing 

 angle is rather large, and even the normal intensity will be 

 \erj small. 



(e) (111) planes. (AEG, B D F, fig. 2.) 



The planes are in pairs, very nearly evenly spaced, the 

 spacing being in the ratio 0*463: 0*537. For the first- 

 order spectrum, the phase-difference between the contribu- 

 tions from the two set of planes is 167°. The first-order 

 spectrum should, therefore, be very small. No first order 

 could be observed at all. The structure assumed approxi- 

 mates to that of a simple rhombohedral lattice, for which 

 the glancing angle for the first- order spectrum for the 

 (111) planes should be double the angle for the face-centred 

 lattice. Thus the spectra from the (111) face confirm the 

 conclusion that the atoms of the second lattice must lie very 

 close to the unoccupied corners of the first lattice, and not 

 somewhere near the middle of the diagonals of the small 

 cells, as would be the case for a structure approximating to 

 that of diamond. 



Accuracy of determination of Position of the Atoms. 



The planes parallel to the (111) face occur in pairs, and it 

 is from the spacing of these planes that the relative positions 

 of the atoms in the crystal are determined. The accuracy 

 with which the spacing ratio of these pairs of planes can be 

 determined depends on the accuracy with which the relative 

 intensities of the spectra of different orders can be measured, 

 on our knowledge of the intensities of the orders of the 

 spectra from a series of equally-spaced planes, and also on 

 the actual spacing of the planes under consideration, since a 



