246 ON THE USE OF SYMBOLS. 



are supposed to proceed along the plane P ; and the 



symbol by which they are to be represented is 



P A P 

 p 



The symbol representing a pair of planes of any 

 particular modification belonging to class d would be 

 (B P B' q B"r : B P B'r B" q ) 



The edges of this primary form are neither per- 

 pendicular nor horizontal, and the decrements by 

 which they become modified might therefore be ex- 

 pressed by the symbols which represent the modifi- 

 cations upon either the terminal or lateral edges of 

 prisms. But as the edges of the tetrahedron are 

 more analagous to the lateral, than to the terminal 

 edges of prisms, the symbol P B P will be used to de- 

 note the modifying planes belonging to classy. 



The Octahedrons. 



The laws of decrement which produce the modify- 

 ing planes of the octahedrons, are, according to the 

 Abbe Haiiy's theory, supposed to take place on paral- 

 lelepipeds, which would be formed by adding two 

 tetrahedrons to two opposite planes of the octahedron. 

 In the appendix to this treatise, rules will be given 

 for determining directly the laws of decrement on the 

 octahedrons, independently of these added tetra- 

 hedrons. And the symbols representing the secondary 

 planes will therefore vary from those adopted by the 

 Abbe Haiiy. 



