FIGURES OF CRYSTALS, 



437 



many cases dispense with the second figure, and draw 

 the directing- planes in pencil, on the same figure 

 which we purpose either to build upon, or to trun- 

 cate. If there should be many planes to be placed 

 on the secondary form, it will be found expedient 

 sometimes to draw the directing lines with the point 

 of a needle only, as the thickness of even a fine pen- 

 cil line may become a source of error. 



Fig. 391. 



Let us suppose an octahedron with a square base, 

 derived from a square prism, mod. a, required to be 

 drawn, so as to envelope the prism from which it is 

 derived; this being the method, as it has been already 

 explained, by which nature is supposed to build up 

 secondary forms. And let the law of decrement pro- 

 ducing the plane, be by 2 rows in breadth, and 3 in 

 height, on the terminal angle. Its symbol would 



consequently be A. 



First draw a small prism of the proportional dimen- 

 sions of the prism we propose to represent; and 

 through the middle of the lateral planes draw the 

 lines h k y i m, parallel to the terminal edges, and 



