FIGURES OF CRYSTALS. 



419 



We may observe that the angle m o n of the upper 

 figure is a right angle, and that the line m n is equal 

 to the line d c of the lower figure. But two lines 

 drawn from the extremities of the diameter of a 

 circle, and touching each other at the circumference, 

 meet at a right angle. It is therefore obvious, that 

 if we describe a semicircle on the line d c as a dia- 

 meter, and draw the chord d i equal to n o, or, which 

 is the same thing, to d h, the line i c will be the re- 

 quired height of the figure. On the perpendiculai 

 from the point h' on the line p <?, take h' o equal to 

 / h of the lower figure, and take o m equal to i c. 

 Join m a"i m c', m b", and the figure is completed. 



To draw the regular, or any of the irregular 

 Octahedrons. 



Fig. 379. 



Draw the square, rectangular, or rhombic base, 



of the octahedron, in the same manner as the bases of 



the prisms of those forms are directed to be drawn. 



Then find the centre of the base a b c, by drawing 



3 G 2 



