[SOMETRIC SYSTEM 



51 



V. Hexahedron; a:ooa:oo a; (ooi), Fig. 80. 



In the previous cases the poles of the most general form have been 

 moved into one of the sides of the triangle, in which it lies; there 

 are -till three possibilities, the three corners of the triangle. Let 



FK,. 7 is. The Trigonal Tris</, t:i- 

 hedron, (hhi). 



Fi<;. 70. Trigonal Trisocta- 

 hedron, (hhi). 



it now be moved, Fig. 73 a, to coincide with the ditetragonal axis, 

 when all eight faces of the most general form grouped around this, 

 as b, b', c, etc., will fall in one plane, producing a form with six 

 ,faces, the hexahedron, or cube, Fig. 80. Each face will cut one 

 axis and is parallel to the other two. The ditetragonal axes will 



FIG. 80. The Hexahedron, (100). 



;. si. The Rimini-it- Dodeca- 

 hedron, (110). 



end in the center of the faces, the ditrigonal in the corners, and the 

 didigonal will bisect the edges. The angles between the faces are 

 fixed at 90, there is but one hexahedron and not a series. It is 

 therefore termed a fixed form. 



