IN TRUE PERSPECTIVE. 
503 
Problem II. To drazv the Rhomhohedral Combination 
R. (P)^. R-hS, of the Species of Rhomhohedral Lime- 
haloide. 
Before all, it is necessary to fix the relative extent which 
the faces belonging to the different forms are meant to pos- 
sess, in order to ascertain which of the simple forms is to be 
projected first, and the others applied to it, according to the 
rules of derivation and of combination. Nothing is more 
easy, if the combination contains only a few simple forms ; 
but it requires some practice to find out the best order in 
which one of the simple forms is to be added after the 
other, if the combination contains a great number of them. 
A short time, however, devoted to the projections of the 
simple forms themselves, is the best assistance for those 
who intend to occupy themselves with representing combi- 
nations. In the present case, it will be most advisable to 
begin with the scalene six-sided pyramid (P)^, and the 
process itself will therefore be as follows. 
Project the pyramid (P)^ or A B C D E FG X, Fig. 11, 
according to the rules given above (Sect. I. Probl. XI.) 
The edges of combination between R and (P)' are paral- 
lel to the opposite terminal edges of the rhombohedron, 
and to the lateral edges of the pyramid. The point in 
which one of the faces of R intersects the edge A C of the 
pyramid, and consequently the ratio of A to A C having 
been determined, it is required to draw the edge of combi- 
nation parallel to the lateral edge C D of the pyra- 
mid, and likewise B^ parallel to C B. By this process 
the ratio of A B^ : AB becomes equal to that of AC^ : AC, 
and so all round, till all the points B^ F^ have 
been determined, and those lines drawn which join these 
points with each other, and represent the edges of combi- 
