256 



RELATION OP DECREMENTS TO 



When the symbolic character is not accompanied 

 by a figure of the crystal, both the preceding- symbols 

 should be given ; but when there is a figure, it will 

 be sufficient to use the second only. 



P * 



B B' B //p P B'" is the symbol representing class /:. 



The regular Tetrahedron. 



Fig. 307. 



Simple and mixed decrements on the angles. 



General symbol P A P . 

 p 



Ifp ^> 1, the symbol represents class b. 



p zz 1, mod. a. 



p < 1, class c. 



Intermediary decrements. 



General symbol, (B P B' q B"r)(B P B'r B" q ) repre- 

 sents class d. 



Decrements on the edges. 



General symbol, P B P . 



1, the symbol represents mod. e. 

 p ^> 1, class f. 



