MODIFICATIONS. 257 



The regular Octahedron. 

 Fig. 308. 



Simple and mixed decrements on the angles. 



General symbol, P A P . 

 p 



If p 1, the symbol represents mod. a. 

 p <^ 1, class b. 



Intermediary decrements 



Are of two kinds, and require two general symbols. 

 1st. (Bp B'q b'q br) represents class c; 

 Id. (B P B'q bV bs : Bp B'r b' q bs) represents class d. 



Decrements on the edges. 

 General symbol, P B P . 



If p zz 1, the symbol represents mod. e. 

 P > 1, class/. 



The planes belonging to classes a and k of the cube, 

 sometimes occur on the same crystal, and when the 

 planes a are much enlarged, the secondary form pre- 

 sents the figure of the octahedron modified by two 

 only of the planes c of that figure. The secondary 

 crystal may however be referred properly to the cube, 

 so long as it retains any portion of the planes A*. 



