On the Drawing of Figures of Crystals. 45 
srow is the plane 2Po (2: o :1); for it cuts off 2 of e and } of 
é, or 2 parts of e to 1 of €. 
The perimeters of the planes untu (2 ‘Po ) and mno (P) intersect 
in the points n and «; the intersections of 2Poa with P has there- 
fore the direction «n, aid is parallel with the edge a : 0’ in figure 10. 
The perimeters of the planes vntu (2Po ) and mro (2P2), inter- 
sect in the pots « and 7; and a line from @ to 7 marks the direc- 
tion of the edge between the planes 2Po and 2P2. 
The perimeters of the planes srow (2Pa ) and mro (2P2), coin- 
cide in the line ro. The intersection of 2Pca and 2P2 has there- 
fore the direction ro and is parallel with the edge 6 : a’ in fig. 10. 
Again, the plane gmol represents P a, (1: 1+ ©) for it cuts off 
equal parts of the edges e and é. The perimeters of the planes 
gmol and nmo (P) coincide in the line mo; their intersection is 
therefore parallel to this line, or to the edges a: 0 and 0 : M, fig. 10. 
The perimeters gmol and npb (4P2) intersect in the points « and 
¢; aline from « to ¢ therefore marks the direction of the edge be- 
tween Po and 4P2 (6’). 
Again, the plane prkh is the projection of oP (am:1: 1), for 
it cuts off equal portions of é and é, and is parallel with the lateral 
edge. The perimeters prkh (oo P) and mbn (2P) intersect in the 
points « and ¢; aline between these points is parallel with mn. 
The intersection of these planes will therefore be parallel with mn, 
or the edge a: a’ (fig. 10.) 
The perimeters prkh (o P), and pnd (4P2) intersect in the soit 
pand¢. A line drawn from p to ¢ determines therefore the inter- 
section of oP and 4P2 (0’). 
Fig. 12. 
Fig. 12, contains these additional planes laid down according to the 
above deductions. ‘The edge a : é/(3Po ) is parallel with the sg 
a: 0’; the edge é ; 6 has the direction “7; the edge 6(2Pa ) : 
