ON THE USE OF SYMBOLS. 233 



are drawn in the preceding tables, the letter A will 

 not necessarily stand on an obtuse angle, excepting 

 on the rhombic dodecahedron, the right rhombic, the 

 right oblique-angled, and the hexagonal prisms. 



The edges and angles of that terminal plane of the 

 prism, on which the figures appear to rest, and the 

 edges and planes which constitute the back of the figures, 

 are supposed to be denoted by a series of small letters, 

 corresponding with the capitals, by which the diametri- 

 cally opposite edges, angles, and planes, exhibited in 

 the front ofthejigure, are designated. 



The representation of the secondary forms of crys- 

 tals by means of these symbols, is effected by annex- 

 ing numbers, expressive of the particular laws of 

 decrement by which the secondary planes are con- 

 ceived to have been respectively produced, to the 

 letters which denote the edges or angles on which 

 the decrements have taken place. 



These numbers will be termed the indices of the 

 secondary planes, and will generally be represented by 

 the letters p q r s. 



Before we proceed, however, to explain the man- 

 ner in which these symbols may be applied to the 

 representation of the secondary forms of crystals, we 

 shall for a moment consider the theory of decrements 

 more particularly in reference to the descriptive cha- 

 racter it affords. 



As this character is to be regarded as little else 

 than a symbol, indicating the change of figure which 

 the primary form has undergone, if there be two laws 

 of decrement which will equally well express this 

 change of figure, we are obviously at liberty to adopt 

 either law, as the generator of the new plane by 

 which the figure of the primary form is altered ; but it 

 will be found convenient to be guided by some rule 

 in our choice. 



