208 Proceedings of Royal Society of Edinburgh. [sess. 
(c) Two parallel nets of atoms at equal distances, 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, SX^/f, 
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 q v q 2 , q% . • • be the radii of the circles shown in fig. 1, 
and r v r 2 , r 3 . . . be the radii of the circles shown in fig. 2 ; and 
for brevity denote X^/f by k. The distances from A of all the 
neighbours around it are : — 
In our “ middle plane ” : 6 each equal to q 1 ; 6, q 2 ; 6, q 3 ; 
12, 2 4 ; 6, ? 6 ; ... . 
