Space Group of a Cubic Crystal. 181 



established for distinguishing in so far as is possible 

 between them. Since in tetartohedral and paramorphic 

 hemihedral (pyritohedral) crystals the plane likl belongs 

 to a different form from kill (for instance 041 is distinct 

 from 401) the Laue photographs to which these crystals 

 give rise will possess less symmetry than photographs of 

 those belonging to one of the other classes of cubic sym- 

 metry. On the basis of an observed hemihedry or holo- 

 hedry in the symmetry of the Laue photographs, crystals 

 of the classes T or T h can always be distinguished from 

 crystals of the classes T d ,0 or O h . 



Crystals corresponding to space groups based upon r c> 

 the simple cubic lattice, will in general like T h 6 give 

 reflections in the first order from planes of all three 

 groups ; those based upon the face-centered lattice, r c ', 

 will give first (or any odd numbered) order reflections 

 only from planes all of whose indices are odd ; and those 

 developed from the body-centered lattice, r c " ? will reflect 

 in the first (or any odd numbered) order only planes 

 having two indices that are odd and one even. Upon the 

 observed symmetry of the corresponding Laue photo- 

 graphs and the nature of the underlying lattice, the cubic 

 space groups can be given the following preliminary 

 arrangement : 



r c : all kinds of planes in all orders, 

 Hemihedral Laue Photographs : 



Tl T4 m i m 2 m « , 

 J - 1 - ? -■- h 7 J- h J - 1 - li f 



Holoheclral Laue Photographs : 



iy : only all odd planes in odd orders, 

 Hemihedral Laue Photographs : 



T2 rp 3 m 4 . 



Holohedral Laue Photographs : 

 T d 2 ,T/,0 3 ,OSO h 5 ,O h 6 ,Oj,O h 8 . 

 r c " : only two odd and one even planes in odd orders, 

 Hemihedral Laue Photographs : 



Holohedral Laue Photographs : 



T^T d 6 ,0 5 ,0 8 ,O h 9 ,O h 10 . 



