504 
ON DRAWING CRYSTALS 
nation between R and {Py, Aline M^N^ parallel to 
D^, or C D, or to M N, the edges of the rhombohedron 
R, and passing through the point B^, denotes the direction 
of one of the terminal edges of R ; its intersection with the 
axis A X, or A^, is the apex of the rhombohedron itself, 
which enters into the combination. The lines A^D^, A^ 
joining this apex with the upper solid angles of the combi- 
nation, complete the figure on this side of the centre ; on 
the opposite side of which the same process must be gone 
through. 
The third form to be combined with the combination of 
R and (P) ^ is R 4- an acute rhombohedron, whose 
edges of combination with (P)^ are parallel to the more 
acute terminal edges of the pyramid. A face of this 
rhombohedron, passing through an arbitrary point O of 
the obtuse terminal edge A C of the pyramid, will there- 
fore produce the edges of combination O P and O Q, 
being the intersections of that plane with those of the 
pyramid ABC, and A C D. In order to find the point S, 
where the face of R -f- 2 intersects the lower opposite edge 
C X, take P R = R Q, and draw O R, which, duly length- 
ened, will intersect C X in S, The face O P S Q will re- 
present one of the faces of R-j-S, as it appears in the com- 
bination. There are six of these faces, three of them in- 
clined to the upper apex, the three others inclined to the 
lower apex ; one of the latter is P^ in the figure. 
But the faces O P S Q and P^ S^Q^ intersect each other 
in the line Z Z^, which line is the lateral edge of R-j-2, 
and, as such, parallel to O P and P^, the edges of com- 
bination between this rhombohedron and the pyramid (P)^, 
or A B and X E the terminal edges of the latter form it- 
self. The result, in completing the combination, by draw- 
ing the four remaining faces of R -f- is the form repre- 
sented in Fig. IS. 
