74 » Dr. L. Vegard on 



In describing the compound lattices it may be convenient 

 to refer the lattices to a rectilinear coordinate system. We 

 let the origin coincide with one of the atomic centres, and 

 the <?-axis be parallel to the tetragonal axis and the #- and 

 ^-axes parallel to the other sides of the lattice. 



Any other elementary lattice which may be made to 

 cover the primary one by a simple translatory movement is 

 completely determined with regard to position by giving the 

 coordinates of one of its points. Very often it is most 

 convenient to give the coordinates of the point nearest to- 

 the origin, which we shall call the point of construction. 



Thus a face-centred lattice is made up of four simple lattices- 

 with the following construction-points : 



(000), (aj2, a/2,0), (0, a/2, c/2), (a/2, 0, c/2), 



and the spacings : 



d m = a/2, aV = c/2, duo = ^ry^> 



The lattice corresponding to the cube-centred lattice has. 

 construction-points 



(000) and (a\2, aj2, c/2), 

 and spacings 



a^o = aft, d m = c/2, d m — -^, 



0*101 = r dm = . . (6 c) 



A lattice analogous to that of diamond is composed of two- 

 parallel face-centred lattices with the construction-points 



(000) and (a/4, a/4, c/4), 



and has the following spacings : 



dioo = a/4, d mi = c/4, duo - 



2 v/2' 

 a 





