Space Group of a Cubic Crystal. 183 



of a single form (the {100} planes) can, however, be used 

 if at all with only the greatest caution because it may 

 readily happen that the scattering powers and relative 

 positions of different atoms in a crystal will be such as to 

 make the reflections from planes of this form so weak as 

 not to be observed under the ordinary conditions of 

 experimentation. Distinctions of this sort have conse- 

 quently been designated as uncertain in the preceding 

 classification. 



The characteristics serving to distinguish between 

 each of the space groups can be stated as follows : 



Space Groups based upon a Simple Cubic Lattice, in 

 general all three kinds of planes appearing in all orders : 



Hemihedral Photographs : 



T 1 and TV : Xo classes of planes absent ; 



T 4 : Planes of the form {100} absent in odd orders ; 



T h 2 : Planes of the form {Old), where k and I are 

 one even and the other odd, are absent in odd orders ; 



T h 6 : Planes of the form {liOl), where k is even and 

 I is odd, and of the form {Okl}, where h and I are both 

 odd, are absent in the odd orders. 



Holohedral Photographs : 



Td 1 , O 1 , and (V : No classes of planes absent. 



T d 4 and O h 3 : Planes of the form {hM}, where h = 

 ± ~k and either li is even and I is odd or both h and I are 

 odd, are absent in odd orders ; 



O 2 : Planes of the form {100} are absent in odd 

 orders ; 



O 6 and O 7 : Planes of the form {100} are absent in 

 all but the fourth, eighth, etc., orders ; 



O h 2 : Planes of the form {old), where Jc is even and 

 I is odd, and of the form {7*7?/}, where either h is even and 

 I odd or both h and I are odd, are absent in odd orders. 



O h 4 : Planes of the form {Old}, where h is even and 

 I is odd, are absent in the odd orders. 



Space Groups based upon a Face Centered Cubic 

 Lattice, in general planes having all odd indices appear- 

 ing in odd orders : 



