TABLES OF MODIFICATIONS. 231 



clination of the modifying plane on both the primary 

 should be given. 



This method of description maybe readily extended 

 to all the classes of primary forms ; and although it 

 may sometimes be rather tedious in its application, it 

 will convey an accurate description of the planes to 

 which it is applied. 



It may frequently happen that we are examining a 

 crystal whose primary form is unknown to us, and 

 whose secondary planes do not enable us to deter- 

 mine that form; we can in such case describe the 

 crystal only by giving a drawing of it, accompanied, 

 by the inclinations of its several planes to each other. 



It will perhaps be found convenient, where it can 

 be done, to number the observed planes, belonging 

 to each class of modifications, in some certain order; 

 when there is a series of secondary planes whose 

 edges are parallel, that plane may be denoted by 

 No. 1, which forms the most obtuse angle with thq 

 primary plane to which the series may be referred. 





