82 Dr. L. Yegard on 



The distance moved being of atomic dimensions, the 

 difference of position of the points II and those of the image 

 of I is not to be detected as long as we regard the properties 

 shown by a large number of atoms. 



Thus this asymmetry should not even be felt by ordinary 

 light waves because one wave-length would cover some 

 thousand atoms ; first when we come to a wave-length of 

 the order of that of Rontgen rays, the want of symmetry 

 with regard to the face (100) can be detected. 



From these considerations we arrive at the following- 

 general rule. 



Fig. 5. 



Arrangement of atoms in 

 (f/o J plane 





N<? N tf\ -\ 



\ , 



,, v 







% Zr atoms 

 O Si » 

 • « 



N 



- ^O 



-- 



Lines of intersection 



with 



(ill) plan 



In order that a certain space lattice shall explain an 

 element of symmetry ascribed to the finite crystal, it will be 

 sufficient that the lattice possesses a symmetry of the follow- 

 ing kind : 



The lattice must be brought to coincidence with itself by 

 performing on it the operation characteristic of the symmetry 

 element in question and a translatory movement x = eia, 

 y = € 2 b, and z = e 3 c, where a, b, c are the sides of the elementary 

 lattice, and e l3 e 2 , e 3 quantities not greater than unity. 



