FIGURES OF CRYSTALS. 



finding successively, the intersections of all the se- 

 condary planes with each other and with the pri- 

 mary. 



Let the primary forms, No. 1 and 2, fig. 384, be 

 drawn in pencil,* and on No. 2 draw u v parallel to 

 the short diagonal of the terminal plane, and from 

 the points u and v, draw u o, v p, parallel to the 

 lateral edges. The plane h would evidently be re- 

 presented by u v op. Let the line a a' be drawn on 

 the plane /z, and through the diagonals q r, s t. 



The plane c should next be placed on the second- 

 ary figure, which plane must obviously lie between 

 h and P. The law of decrement from which we 

 have supposed this plane to result, is by 2 rows 

 in breadth. If therefore we produce two of the pri- 

 mary terminal edges of No. 1 to c and c', so that q c, 

 and q c', shall each be double the edge that is pro- 

 duced; and if we join c c', the plane a c c', will 

 represent the plane c, the line c c' will touch the 

 primary form at the solid angle d, and the line da 

 will pass through the middle of the plane a c c'. 



We may call d a, therefore, the directing line, 

 which enables us to place the plane c between h and 

 P, by taking some point d, in the diagonal q r, No. 2, 

 and drawing d a parallel to d , No. 1. Through d 

 and 0, we may now draw the lines g /*', i k, parallel 



* When two or more figures are to be drawn in the relation to each 

 other in which these stand, their dimensions should be similar, and 

 their corresponding edges or diagonals should either be parallel or ih 

 the same line, according to the relative positions of the figures. 



In the above figures, the corresponding lateral edges are in the same 

 right lines, and the terminal edges are respectively parallel. These 

 < ot responding positions are always implied when two or more analo- 

 gous figures arc jnveii. 



3 u 2 



