438 APPENDIX ON DRAWING THE CRYSTALS. 



also draw the prismatic axis,//. The dimensions 

 of the circumscribing- figure should be such, that 

 its planes should touch the solid angles of the con- 

 tained prism. 



The directing plane, b c d, evidently represents a 

 plane derived from the modification we have sup- 

 posed ; and the line e d passes through the middle of 

 the plane. If, therefore, we draw gf parallel to d e, 

 and touching the solid angle of the prism at , we 

 obtain the pointy, at which the secondary plane cuts 

 the prismatic axis. From the middle of the axis at o, 

 take o I, equal to o/, and draw / ?z, touching another 

 solid angle of the prism. The point n will evidently 

 be on an edge of the secondary figure, which edge 

 we know must be parallel to a diagonal of the ter- 

 minal planes of the prism. 



Through the point n, therefore, draw h i parallel 

 to one of those diagonals ; from the points h and i 

 draw h m, i , and join k m to complete the base of 

 the figure. And the terminal edges being drawn, the 

 entire figure will result, as shewn by the dark lines 

 in the engraving. 



