182 TERMINOLOGY. . 14:8. 



tion of rhombohedral Emerald, designated indeterminately 

 thus : 



R os. P + n. P + n 1 . 2 (II + n). P + cc. 

 aid c e 



Here R 03 (a) and P 4- co (e) the limits of the series of 

 isosceles six-sided pyramids are immediately determined. 

 The only rhombohedron which it contains is R + n 11 (c). 

 It is present in both positions ; the combination therefore 

 assumes a di-rhombohedral character (. 146.). If this 

 rhombohedron be considered as the fundamental form, the 

 value of n 11 wiU be = 0, and 2 (R + n" ) therefore = 2 (R). 



The faces of P + n (&) if duly enlarged, appear as 

 rhombs in the place of the apices of the di-rhombohedron ; 

 the faces of the latter likewise would be rhombs, were they 

 not intersected by the faces of other forms. Hence n is 

 also = 0, or the pyramid P + n, and the di-rhombohedron 

 2 (R) are co-ordinate forms (. 146. 1. ; and . 145. iii. 1.). 

 P + n therefore is = P. 



The faces of the di-rhombohedron appear with parallel 

 edges of combination in the place of the terminal edges of 

 P + n 1 (d). The relations of the pyramid and the rhom- 

 bohedron will therefore be those considered in . 145. iii. 2. 

 From these it follows that n 1 is = n 11 + 1 = 4- 1 = 1. 

 The pyramid will be = P + 1. 



The determined designation of the developed di-rhombo- 

 hedral combination is therefore : 



R oo. P. 2 (R> P + 1. P + os. 



a be d e 



These developements, as represented by the signs, con- 

 tain every thing required for calculations referring to the 

 compound forms ; since the designation contains all those 

 determined definite values of m and n, which must be sub- 

 stituted in the general equations referred to in . 144. 



The developement of the combinations is peculiarly ap- 



