ON THE USE OF SYMBOLS. 237 



The crystal is supposed to be held with the plane 

 marked P, horizontal, and with that edge or angle 

 nearest to the eye on which the decrement we are 

 about to describe has taken place. 



Let us suppose this crystal to be modified by an 

 individual plane, belonging to the series of modifi- 

 cations of that figure comprehended under class b. 



The planes belonging to this class of modifications, 

 may incline more or less on either of the adjacent 

 primary planes, and may result from a decrement on 

 either of the adjacent plane angles which constitute 

 the solid angle on which O is placed. 



If the modifying plane be produced by a simple or 

 mixed decrement, beginning at the angle O, and 

 proceeding along the terminal plane, consisting of 

 one row of single molecules, it should be expressed 



thus, O, and be read, one over O, signifying that the 

 abstraction of molecules from the superimposed plates 

 took place above, or receding from O, in the direction 

 O A. If the decrement be simple, and by two rows 

 of molecules in breadth, it would be expressed by 



O, and if it be a mixed decrement by three rows in 



4 

 breath and two in height, it would be denoted by O, 



and so of any other decrement acting in that di- 

 rection. 



Jf the modifying plane be occasioned by a simple 

 or mixed decrement, beginning at the angle of the 

 plane M adjacent to O, and proceeding along the 

 plane M, by p rows of molecules, p signifying any 

 whole number or fraction, it would be denoted by 

 r O, and be read p on the left of O. 



If the new plane were produced by a simple or 

 mixed decrement by p rows on the angle of the plane 

 T, adjacent to O 5 and proceeding along that plane, it 



