278 RELATION OF DECREMENTS TO 



represents; and it is directly indicated in each branch 

 of the second symbol, by those indices which denote 

 the abstraction of equal numbers of molecules in the 

 direction of the edges D' b", and D b". 



The Abbe Hauy has referred some of the planes 

 which modify the lateral solid angles, to the angle at 

 E, and others to the angle at Q. It may therefore be 

 convenient to possess the symbols representing those 

 planes, in reference to both angles, and they will 

 accordingly be given below. The symbol on the left 

 represents the modification when the decrement is 

 referred to the angle at O, and that on the right 

 represents the same modification when the decrement 

 is referred to the angle at E. 



1. Simple and mixed decrements on the lateral solid 

 angles. 



1. Producing one plane on each solid angle. 



General symbols. 

 In reference to angle C. 



q 



In reference to angle E. 



(Dp Bq D"p). 



If p zz 2 q, in either of these symbols, mod. e is 



represented. 

 P <C ^ #> cfaw 



J. Producing two planes on each solid angle. 

 General symbols. 



P O 1J . 

 or, 



E p 



\Jly 



(D' P b" P D q : Dp b"p D' q ). 



These symbols represent the series of planes be- 

 longing to class h. 



