710 Proceedings of Royal Society of Edinburgh. [sess. 
In fig. 2 the points PQOP'Q' and their congeners represent a 
homogeneous distribution in one plane. The orthogonal projection 
on this plane of the points in the two nearest parallel planes are 
represented respectively by R and its congeners, black dots ( • ), and 
by R' and its congeners, white dots ( 0 ). Thus explained, the dia- 
gram (fig. 2) is a complete specification of the whole homogeneous 
assemblage throughout space. 
(h) Tetrahedronal Grouping . — Choose any one of the triangles 
(OPQ), and any point (S) in the nearest plane of points on either 
side of it ; and imagine a tetrahedron of which these (OPQS) are the 
four corner points. By similarly dealing with all the triangles of 
all the planes, con-orientational with the first chosen triangle, and the 
points corresponding to the first chosen point in the neighbouring 
plane, we form a homogeneous assemblage of equal homochirally 
similar, samely oriented, tetrahedrons. Thus, for example, take the 
triangle FGH which is con-orientational with QOP. The tetrahedron 
on the base FGH corresponding to SQOP is RFGH. 
Each point of the distribution is the common corner point of 
