TA15LES OF MODIFICATIONS. 229 



our choice would probably fix on that which was 

 most predominant among the secondary forms. 



If a crystal is to be described by the assistance of the 

 preceding tables, we must suppose the primary form 

 to be known ; this must be first described according 

 to its class, and if necessary by its angles also. 



Its modifications, if they are single, may then be 

 denoted by the letters under which they are arranged 

 in the tables. But as each of the classes of modifications, 

 except those which consist of tangent planes, compre- 

 hends an almost unlimited number of individual modi- 

 fying planes, differing from each other in the angles at 

 which they respectively incline on the primary planes, it 

 becomes necessary to add to the tabular letter which 

 expresses the modification, the value of the angles at 

 which the plane we have observed inclines on the 

 adjacent primary planes. 



We have already seen that modification a of the 

 right rectangular prism, comprehends a considerable 

 number of planes varying in their relative inclinations 

 on P, M, and T. Let us suppose the crystal we are 

 examining, to belong to the class of right rectangular 

 prisms, and to be modified by a plane a, and let the 

 inclination of the plane we have observed, on P, M, 

 and T, be called m, n*^ and o", these letters signifying 

 any number of degrees and minutes whatsoever. 



A crystal containing the primary planes of the 

 right rectangular prism, and a set of planes belong- 

 ing to modification a might then be thus described. 



Right rectangular prism, Modification a, m on P. 



n M. 

 o T. 



The character of the plane being thus established, 

 Vve may in future, in order to avoid the repetition of 



