FIGURES OF CRYSTALS. 



Fig. 386. 



431 



Let P, P' P", represent the primary planes. 



The planes o maybe added, by merely lengthening 

 the primary axis, drawing at its two extremities the 

 three upper and three lower primary planes of the 

 rhomboid, and joining their angles by six vertical 

 lines, which will then constitute the vertical edges 

 of the plane o. 



To add the planes m, take any points c c', on the 

 two adjacent edges of the rhomboid, such that a line 

 passing through both should be parallel to the hori- 

 zontal diagonal of the plane P" ; and from these 

 points draw lines on the planes P and P', parallel to 

 their common edge. 



The intersection of the planes m with each other, 

 is parallel to a line passing through two opposite 

 solid angles, and through the axis. 



Fig. 387. 



This is apparent from fig. 387, in which if the 



