FIGURES OF CRYSTALS. 421 



portions of the lines, to the solid angles of the cube. 

 The resulting figure will be the rhombic dodeca- 

 hedron. 



In the preceding pages the rules of projection 

 have been applied to the delineation of only the 

 primary forms of crystals ; but they may also be ap- 

 plied to the delineation of some of the secondary 

 forms ; these may however be more easily drawn 

 either by truncating the figures of the primary forms, 

 or by circumscribing those primary forms with the 

 planes of the secondary crystal. 



When the secondary form, whether it be simple or 

 compound, is to be exhibited in its entire state, with 

 all the primary planes effaced, the best method will 

 be to delineate a small primary form, and to envelope 

 that with the secondary planes ; but when parts of 

 the primary planes are also to be shewn in the figure 

 of the secondary crystal, a larger primary form may 

 be drawn, and then be truncated, or cut down, in the 

 same manner as the modifications in the tables are 

 drawn. 



The fidelity of the representation of any secondary 

 form, must obviously depend upon the accuracy of* 

 the positions of the secondary planes relatively to the 

 nrimary and to each other. And as it is by the in- 

 tersections of those planes with the primary or with 

 each other, that their positions are rendered appa- 

 rent, the accurate construction of a secondary form, 

 must depend upon the accuracy with which those 

 intersections are determined. 



Hence the rules for drawing the secondary forms 

 of crystals, will apply chiefly to the means of finding 

 the intersections of the secondary planes. 



There are two principal methods which may be 

 used for this purpose ; the one is to divide the edges 



