240 ON THE USE OF SYMBOLS. 



rows of molecules proceeding along the plane P ; p, 

 meaning^ as before, any whole number or fraction. 



The symbol to denote this decrement, would be F, 

 and be read as before, p over F. 



If the decrement be supposed to have proceeded 

 along- the plane T by three rows of molecules, as in 

 figure 303, the general symbol used to represent the 



new plane would still be F, but p would in this case 

 represent the fraction y, and the particular symbol 



T 



would be F. 



If we suppose p to be a fraction , it is evident from 

 what has been already slated, that the numerator of that 

 fraction may be either greater or less than the denomi- 

 nator^ according as the decrement in breadth exceeds or 

 falls short of that in height. ' 



If either of the other terminal edges be modified, 

 the modified edge is supposed to be the nearest to 

 the eye, when the modifying plane is described. 



This change of position must be understood to 

 take place in every instance where the position of 

 the modified edge or angle requires it. 



If two dissimilar planes occur on the same terminal 



p P 

 edge of a crystal, the symbol is repeated thus F F, 



which expresses the coexistence of the two planes on 

 one edge. 



If the lateral edge H of a doubly oblique prism be 

 modified, and if it has been found that the decrement 

 producing it has proceeded along the plane M, by p 

 rows of molecules, its characteristic symbol would be 

 1} H, and it would be read^? on the left of H. 



If the decrement appears to have proceeded along 

 the plane T by p rows of molecules, its symbol would 

 be H p , or p on the right of H ; p being either a 



