SYMMETRY OF SECONDARY PLANES. 



35 



. 50. DISPOSITION OF SECONDARY PLANES REGULATED 

 BY SYMMETRICAL LAWS. 



The same modification generally takes place upon every 

 similar edge or angle of a primary form. 



In the Tetrahedron, the Cube, and the regular Octahedron, (forms, 

 among whose edges and angles there exists the most perfect similar- 

 ity,) when one edge or angle is subject to a replacement, we find all 

 similarly affected. The following figures, representing actual crys- 

 tals, illustrate this idea. 



Fig. 52 exhibits the Tetrahedron, with its solid angles replaced by 

 tangent planes. 



Fig. 52. 



Fig. 53. 



Fig. 54. 



Fig. 53, the same, with its edges replaced by tangent planes. 

 Fig. 54, the same, with its angles replaced by three planes, rest- 

 ing on the primary planes. 



Fig. 55. Fig. 56. 



Fig. 55, the same, with its angles replaced by three planes, resting 

 on the primary edges. 



Fig. 56, exhibits the Cube with its angles replaced by tangent 

 planes. v 



