276 RELATION OF DECREMENTS TO 



(Bp Gq BV : Br Gq B'p) represents those planes 

 of class b which are produced by 

 intermediary decrements. 



13 represents class c. 

 1 G 1 . . . mod. d. 

 P G P . . . class e. 



In some crystals of phosphate of lime, the planes 

 belonging to class b occur singly. If they result from 

 simple or mixed decrements, their symbol would be 

 A P or P A, according as they lie on the left or right 

 of the modified angle. And if they are produced by 

 intermediary decrements, their symbol might be 

 (Bo Go B'o : B q G P B'r.) 



The Rhomboid. 

 Fig. 320. 



Simple and mixed decrements on the superior angles. 



General symbol P A P . 

 p 



Ifp n: 1, the symbol represents mod. a. 

 p ^> 1, . . . . . . . . class b. 



p <^ J, class c. 



