ON THE USE OF SYMBOLS. 239 



A is placed is nearest to the eye; or we may be sup- 

 posed to pass round the crystal, until we place our- 

 selves opposite the angle at A ; and while the eye and 

 that angle are in these relative positions, we should 

 proceed to describe the new planes, as we did those 

 on the solid angle at O. 



If two or more planes, resulting from simple or 

 mixed decrements, are found modifying the same 

 solid angle of any crystal, the symbols representing 

 them are to be placed immediately following each 

 other. Thus if the three planes we have supposed on 

 the angle O, should occur on the same crystal, its 

 change of figure would be thus represented, 



p o 6 o p . 



These symbols not being placed in a parenthesis, are 

 understood to represent three separate planes. 



If three intermediary decrements should occur on 

 the same solid angle, their symbols would also be 

 placed following each other, thus, 



(D3 H2 F4) (Dl H3 F2) (D4 HI F3). 



Here, each of the three sets of characters being 

 included within a separate parenthesis , three varieties 

 of intermediary decrement are implied ; and as they 

 stand singly , it is implied that they are independent of 

 each other-, and they are evidently produced by dif- 

 ferent laws of decrement. 

 



Let us now suppose we are about to describe a 

 decrement on a terminal edge of a doubly oblique 

 prism. The prism is again supposed to be placed or 

 held with that edge nearest to us, the plane P con- 

 tinuing horizontal. 



And first let us suppose a terminal edge F to be 

 replaced by a plane resulting from a decrement by p 



