FIGURES OF CRYSTALS 



423 



The other corresponding secondary edges being 

 drawn, a secondary form will result, as exhibited by 

 the dark lines in the above figure. 



Fig. 382. 



Let it be required to draw a right rectangular 

 prism, with two planes belonging to class a modi- 

 fying each of the solid angles. The dimensions of 

 the prism are supposed to have been ascertained, 

 and a prism is to be drawn in pencil corresponding 

 in relative dimensions with our observed crystal. 



We shall suppose the symbols of the two secondary 

 planes to have been found, 



(2 Gi 63) and (Ci G$ 62). 



Let each of the edges be divided into 7 equal 

 parts ; the plane represented by the first symbol 

 would be traced by the lines a #, a c, b c, and that 

 represented by the second would be traced by the 

 lines fh) h g. But it is apparent that the two planes 

 would intersect each other in the line d e, and that 

 only the portions a d e c, and d e h, of the new planes, 

 would be visible upon the secondary crystal. The 

 same process being repeated on the other solid 

 angles, the modified form, as shewn by the inner lines 

 of the figure, would be produced. 



