ON THE USE OF SYMBOLS. 251 



plane, while the other remains entire, the symbol 

 representing the change of figure would be 



3. When different decrements take place on two 

 similar angles or edges, the number expressing the 

 law of one of them may be represented by ^?, and the 

 number representing the law of the other, may be 

 substituted for the o, by which we have proposed to 

 denote the unmodified angles or edges. Thus, if one 

 terminal solid angle of a rhomboid were replaced by 

 a tangent plane, and the other by three planes be- 

 longing to class &, resulting from a decrement by two 

 rows in breadth, the symbol would become 

 'A 1 2 a 2 . 



1 2 



It will be convenient when we describe the second- 

 ary forms of crystals by means of these symbols, to 

 observe some certain order in their arrangement into 

 what may be termed the theoretical image of the 

 crystal. The Abbe Haiiy places the symbols re- 

 presenting the lateral edges of prisms, first; then 

 those which represent the terminal edges; and lastly, 

 those which represent the solid angles. As it is evi- 

 dently indifferent whether they be taken in this order 

 or in any other, as far as their descriptive character 

 is concerned, I shall observe the same order of ar- 

 rangement that he has given ; although if that had 

 not been established, I should have reversed it, for 

 the sake of conformity with the order in which the 

 modifications are placed in the tables. 



The Abbe Haiiy has also proposed to designate the 

 secondary planes by small letters, and to place these 

 under the respective symbols of the planes they refer 

 to. And in order to render the character of the sym- 

 bol more complete, he repeats the letters which 

 designate the primary planes, among those which 



