234 



ON THE USE OF SYMBOLS. 



Fig. 302. 



If, for example, we find a secondary plane, such as 

 a b, fig. 302, on the terminal edge of any prism, pro- 

 duced by the abstraction of three rows of molecules 

 in the direction of the lateral edges, and of one row in 

 the direction of the terminal edges, such a plane 

 might be conceived to be produced by a decrement 

 proceeding along the plane a c, consisting of three 

 molecules in height and one in breadth, or of three 

 molecules in breadth, if we suppose it to proceed along 

 the planes b d; and the symbol denoting either of 

 these decrements, might therefore with equal pro- 

 priety be used to describe the new plane; and it 

 would be indifferent, as far as the descriptive charac- 

 ter of the symbol is regarded, which of the two we 

 should adopt. 



The rule which it will be more convenient to fol- 

 low, is, to suppose all planes on the terminal edges of 

 prisms, to be produced by decrements proceeding 

 along the terminal planes ; and the planes replacing 

 the lateral edges of prisms, and the edges of all the 

 other classes of primary forms, may be conceived to 

 result from decrements proceeding along those planes 

 in the direction of which the greatest number of 

 molecules appear to have been abstracted. And any 

 intermediary decrement may be conceived to proceed 



