FIGURES OF CRYSTALS. 



411 



character; that obliquity is therefore considerably 

 increased in the" following figure. 



Fig. 373. 



As the lateral angles of this form are not right 

 angles, its base obviously cannot rest on a horizontal 

 plane, while its lateral edges are perpendicular. 

 To obtain its horizontal projection therefore, while 

 its lateral edges are perpendicular, we may suppose 

 those edges produced until they touch the horizontal 

 plane i m, over which the figures appears to stand. 



The area of the horizontal projection is clearly 

 less than the base of the figure, and may be known 

 from the ratio of the terminal edges, and from the 

 plane angles of the lateral planes ; which elements 

 are supposed to have been previously determined. 



SF2 



