86 





Dr. L. Veg 



ard on tlie 





Crystal 

 plane. 



K 



d. 



Eelative Intensity. 



1. 



2. 



3 order. 



(100) 



8° 36' 



cm. 

 2-030 10~ 8 



100 



0-25 



007 



(110) 



12° 11' 



1-438 „ 





Not observed. 



(111) 



7° 27' 



2-341 „ 



1-00 



049 



010 



The faces (100) and (111) show a normal* variation of inten- 

 sities, with the intensity rapidly decreasing with increasing- 

 order. 



For the ratio of the grating constants we get 



*f=* =1-1533 1 , 



"100 I «1 



4= =M547 J 

 ^-° =0-7085), 



"100 V ^110 — 1- 



-in= * 

 ^100 v*^ 



s/2 



= 0-7071 



'100 



x/2 



These are the well-known ratios, which belong to the face- 

 centred cubic lattice * (fig. 2). For if the side of the smallest 

 cube with one atom in each corner is 2a, then we shall have 



"ioo — a t d 



no 



a 



, 2a 



dm- yg 



Thus the ratios of the grating constants are explained from 

 the face-centred lattice. This arrangement would further 

 explain the normal distribution of intensities, as all point 

 planes parallel to one of the faces examined should be identical 

 and equidistant. 



To put the lattice to a final test we shall calculate from 

 observations the number (n) of atoms which are associated 

 with a cube of side d m . 



In the volume of the whole cube lattice with side 2a 

 (fig. 2) there are 4 atoms, and in the cube with a side a = d m 

 there should be J atom on an average. 



For the number n we set 



n— ^-Ndfoo- 

 * W. H. Bragg and W. L. Bragg, Proc. Roy. Soc. vol. lxxxix. p. 281. 



