IN TRUE PERSPECTIVE. 
501 
of the relations of the forms themselves, and of the rela- 
tions they bear to each other. It will be useful, however, 
to illustrate, by a few examples, the method of drawing the 
figures of combinations. 
Section II. 
Of the Peojections of Compound Forms. 
Compound forms or combinations, in general, are those 
crystalline figures, which at the same time shew the faces 
of two or more simple forms. Every one of these may be 
obtained by sufficiently enlarging those faces which, in the 
compound form, are equal and similar to each other. The 
form of the combination may be defined to be the space 
included at the same time within all the forms entering 
into the combination. Hence the method of representing 
them in its greatest generality, will require to lay down, in 
the parallel position, the figures of all those simple forms 
which the combination contains, and to determine that part 
of them which, if they intersect each other, is not excluded 
by any one of these simple forms. In most cases we may 
dispense with proceeding upon this long and very often 
tedious way, but it will be necessary to shew it in an ex- 
ample. 
Problem I. To draw the Combination of the Hexahedron 
and the Octahedron. 
Project the two simple forms in parallel position, so that 
their centres may coincide in M, Fig. 10. 
If we draw EF parallel to DB, and IK parallel to LP, 
through the centres of the respective edges of the hexahe- 
