2 ———————— 
On the Drawing of Figures of Crystals. _ 49 
and therefore are not in the line of the rhombic interaxes. ‘The 
. sabes [mOn] 
coordinates of this solid angle for any form, as ——g_? may be found 
m(n—1) n(m— 1) 
m1 ina 1 
las, the situation of two points, a 
and 5, (fig. 14.) in each of the 
axes may be determined: and if 
lines are drawn through a and 6 
in each semiaxis parallel to the 
other axes, the intersections c¢, c’ 
of these lines will be the vertices 
of the unsymmetrical solid angles, 
[mOn| 
those marked c of the form ito 18 
by the formulas By means of these formu- 
and those marked c’, of the form 
[mOn] 
Te 
The trigonal interaxes are of the same length as in the holohedral 
forms. ‘The values of these interaxes, and of the coordinates of the 
unsymmetrical solid angle for different parallel hemihedrons, are con- 
tained in the following table. 
Trigonal Coord. of the un- 
interaxis. sym. solid angle. 
303 
Parallel hemihexoctahedron (fig. 49.) —- = 4 A 
6c [402] 5 4 6 
5) 7 7 7 
ce [503] 2 4 10. 
5) 3 Aas ep 
aO2 
Hemitetrahexahedron (fig. 44.) ae “ a 1 
aO02 
« 5 Lia cag 
ih [ 003] iM " i 
2 4 3 
24. Dimetric system.—In an octagonal pyramid, mPn (fig. 59.) the 
interaxes, or diagonals symmetrically intermediate between the hori- 
zontal axes, terminate in the interaxal basal angles. Their length ex- 
ceeds the length of the interaxes of the octahedron, by a portion equal 
Vou. XX XIJI.—No. 1. 7 
