224 MR HAIDINGER on the Forms of Crystallisation 



horizontal, the form to which the latter faces belong is the limit 

 of the series of horizontal prisms, contiguous to the long dia- 

 gonal, and is consequently designated by Pf + oo . The sign 

 of the whole combination is 



a P e P <> c b 



Another combination is represented in Fig. 2. Besides those 

 of the preceding variety, it contains the faces d,f, g, g', and h. 

 The faces d form oblique truncations of the edges between b and 

 c, and they are inclined to the former at an angle of 156 27' ; if 

 enlarged to their intersection above these faces, they will meet 

 under an angle of 132 54', while that of the faces c and c is 59 

 40'. The diagonal c being supposed equal in both prisms, b' of 

 the prism d will be % b of the prism c. The edges of combina- 

 tion between a and h are paraUel to those between h and d; the 

 transverse section of the scalene four-sided pyramid produced by 

 the enlargement of the faces h, is therefore similar to that of the 

 prism produced by the faces d. But the edges of combination 

 between P and h are parallel to those between k andf,f and h, 

 and h and P, and consequently to the acute terminal edges of P ; 

 the ratio of ' and b', or of the axis, and that diagonal of the base 

 which corresponds to the long diagonal of P, will be in the de- 

 rived pyramid the same as that of a and b in the fundamental 

 form. The ratio of the three lines a' : b' : c' of the pyramid A 



may be expressed by a : b : 4 c, which is equal to - ? : b : 4 c. 

 In the method of crystallographic designation of MOHS, the sign 

 (p") m i s given to a derived pyramid, one of the diagonals of 

 which is equal to the similarly situated long diagonal of the 

 fundamental pyramid P', while the other diagonal and the axis 

 are equal to m times the analogous lines in that form. In the case 



