ON THE DIRECT DETERMINATION OF 

 THE LAWS OF DECREMENT FROM THE 

 PARALLELISM OF THE SECONDARY 

 EDGES OF CRYSTALS. 



THE resources of crystallography for determining 

 the laws of decrement by which secondary planes are 

 produced, are not limited to the methods already 

 explained. In certain cases those laws may be deter- 

 mined, independently of the angle at which the 

 secondary plane inclines on the primary, by means of 

 the parallelisms which are observed to exist between 

 two of the edges of the secondary plane, and two 

 other known edges of the crystal ; or sometimes one 

 known edge, and a diagonal of the primary form. 



One of the simplest instances of the distinctive 

 character conferred on a secondary plane by the paral- 

 lelism of its edges, is that by which the planes 

 produced by simple or mixed decrements are dis- 

 tinguished from those produced by intermediary 

 decrements. 



We may also refer for an illustration to the tables of 

 modifications of the right square prism. The paral- 

 lelism of the lateral edges of the plane c?, to the 

 lateral edge of the prism, implies that plane d is pro- 



3D 



