IN TRUE PEUSPECTIVE. 
499 
The projection of a regular hexagonal prism, which is a 
member of the rhombohedral system, allows of a similar 
application in regard to forms of the same system, as the 
projection of the hexahedron in the forms of other systems. 
Problem X. To draw a Rhomhohedron. 
Let, for instance, this rhomhohedron be the one whose 
terminal edge is = 104° 28' W, the same which Abbe 
Haiiy considered as the primitive form of calcareous spar. 
Its axis is = 1.5 — 2.25. ' 
Draw the regular hexagonal prism, whose sides are 
squares, in the position fixed upon, Fig. 5. Produce the la- 
teral edges, till A A^^ is =: | A A^, equal to the given axis 
of the rhomhohedron. Take one-third of the length of these 
lateral edges alternately from the upper and the lower 
hexagon, and join the points A"^ B^, C"^ D^, E"^ 
thus determined among each other and with the centres 
M, of the adjacent terminal hexagons by straight lines 
The result will be the projection of the given rhomho- 
hedron. The projection of the actual fundamental rhomho- 
hedron of rhombohedral lime-haloide is obtained, if, in- 
stead of taking A A" = | A A^, we substitute the value 
= V^.1895 AA^ 
If M or A A" is = A A^ . a/4 5, the projected solid 
is the hexahedron. 
If in Fig. 2., is — 0, the method of drawing a 
rhomhohedron becomes very simple, since it requires only 
to draw the vertical lines representing the projections of 
the lateral sides and the axis of the hexagonal prism at the 
regular distances (at equal distances, for A^ P^ = J A^ in 
Fig. 2.) from each other, and cross them at right angles 
by four equidistant horizontal lines, whose intersections 
I i 2 
