' axis be connected with the points E or 
On the Drawing of Figures of Crystals. Al 
_ hexagonal plane with the extremities of the vertical axis, a second 
isosceles dodecahedron is projected. 
16. To construct a rhombohedron, 
lay off verticals through the extremi- 
ties of the horizontal axes, and make 
the parts, both above and below these 
extremities, equal to the third of the ; 
vertical semiaxis, (fig.8.) The pomts , |. 
E, E, E/ E/ &c. are thus determined ; | 
and if the extremities of the vertical 4 
E’, rhombohedrons, in different po- 
sitions, mR or —mR, will be con- 
structed. | 
Delineation of Secondary Planes on the Primary Forms. 
17. Previous to drawing the secondary planes on a primary, it 
becomes necessary to determine the direction of the intersections of 
these planes with the primary faces, and also in most cases, with other 
secondary planes. The principles of analytical geometry have af- 
forded Naumann formulas for these intersections; but it would be 
giving this article too great an extension to enter into a full discus- 
sion of this method of determining intersections. It is in general 
sufficient to employ the method of construction. This method has 
been fully explained by Brooke, but in connection with the Abbé 
Haiiy’s system of crystallographic notation. 
In the employment of the plan of construction, the projection of 
the prism OP. aPo , is the most convenient preliminary step; that 
is, the cube in the monometric system, right square prism in the di- 
metric system, the rectangular prism in the trimetric, the right 
rhomboidal in the monoclinate, and the prism OP.c> Pa . Po, in 
the diclinate and triclinate systems.* This is advisable because in 
these forms the lateral edges are equal and parallel to the vertical 
axis, and the basal edges, to the horizontal axes; and consequently 
in laying off the different planes, these edges may be substituted for 
the axes. 
* The system of notation here adopted, is that employed by Naumann. It will 
be found explained and illustrated in my system of Mineralogy. 
Vou. XXXIII.—No. 1. - 6 
