ITS MODIFICATION'S. 1J9 



Class d. Solid angles replaced by eight planes. 

 The new figures would be contained within 48 

 triangular planes. 



Class e. Edges replaced by tangent planes. 



The new figure would be the rhombic dodecahedron. 



Plane e on P or P', 144' 44' 8". 



Class/. Edges replaced by two planes. 

 The new figures would present a trihedral pyramid 

 on each primary plane. 



It has been already stated that the cleavages of the 

 octahedron and the tetrahedron are perfectly similar, 

 and that it is only by means of the secondary planes 

 of each, that we can discriminate these primary forms 

 from each other. 



The octahedron may be distinguished as a primary 

 form from the tetrahedron, according to the rules 

 laid down in a former section, by such of its single 

 modifications as could be produced on the tetra- 

 hedron only by the simultaneous operation of two or 

 more separate modifications. 



But it is obvious that all the modifications of the 

 octahedron, except mod. a and (/, would require a 

 double decrement for their production, supposing the 

 tetrahedron were the primary form. 



