CHAPTER II 

 CRYSTALLOGRAPHY 



Drawing of crystals. The object to be attained in the drawing 

 of crystals may be either to represent their relation and habit in 

 perspective, or to represent the relation of forms on individual crys- 

 tals. The methods in use for this purpose are those of general 

 projection, though modified in some cases to fit the conditions. 

 The edges of crystals are formed in every case by the intersection 

 of two faces ; in the drawing they are represented by straight lines ; 

 to find the position, inclination, and foreshortening of any edge 

 is nothing more than a problem in the intersection of planes. The 

 position of each face is given by the crystallographical symbols. 

 Interfacial angles are used in the drawing, only in so far as they 

 are necessary to determine the axial ratio and intercepts. The 

 methods of perspective projection are modified, not only to simplify 

 the construction, but to adapt it to the representation of crystal 

 edges, so that edges parallel on the crystal will be parallel in the 

 drawing, and of a length proportional to their actual length on the 

 crystal. This modification is simply placing the eye at infinity ; all 

 rays will then be parallel. All parallel and zonal directions will be 

 preserved ; it will be necessary to determine the direction of only 

 one edge of a zone in the drawing, all other edges in the same zone 

 will be parallel. It is customary to represent the ideal symmetry 

 of a crystal in the drawing, unless it is wished to illustrate some 

 peculiar development or habit. There are two general methods of 

 projection, the orthographic and clinographic methods, both of 

 which place the eye at infinity. 



Orthographic projection is a plan or map of the crystal faces and 

 edges, drawn on a plane perpendicular to the c axis. It represents 

 in crystallography exactly what the foundation and roof plans of a 

 house do to a builder. When the plane of projection is perpendicu- 

 lar to the c axis the eye will lie in the direction of the c axis, at an 

 infinite distance, vertically above the plane ; the c axis will appear 



31 



