Molecular Dynamics of a Crystal. 1-41 



drawn perpendicular to it through the atom A, and the points 

 marked b, c, to guide the placing of nets of atoms in parallel 

 planes on its two sides. 



(b) Two nets of atoms at equal distances \\/i on the two 

 sides of the " middle plane/' These nets are so placed that 

 an atom of one of them, say the near one as we look at the 

 diagram, is in the guide line b ; and an atom of the far one 

 is in the guide line c. 



Fig. 1. 



(c) Two parallel nets of atoms at equal distances, 2\ %/§, 

 on the two sides of the " middle plane," so placed that an 

 atom of the near one is in the guide line c, and an atom of 

 the far one is in the guide line b. 



(d) A third pair of parallel planes at equal distances, 

 3Xi/§, from the "middle plane," and each of them having 

 an atom in guide line A. 



(e) Successive triplets of parallel nets with their atoms 

 cyclically arranged Abe Abe . . at greater and greater 

 distances from A on the near side of the paper, and Acb 

 Acb . . at greater and greater distances on the far side. 



§ 7. Let g ly g 2i q, ... be the radii of the circles shown in 

 fig. 1, and r l5 r 2 , r s . . . be the radii of the circles shown in 



