196 TERMINOLOGY. . 152. 



i. P + n. P 4- n'. 



The bases of the forms contained in this combination 

 are similar to each other, because these forms are members 

 of one and the same series, and no different position can 

 have any influence upon them, whatever members of the 

 series may be combined. The edges of combination, there- 

 fore, become horizontal for every value of n and n', even 

 though this be = 4- co or = co. The same reasoning 

 applies to all such combinations as are produced by simple 

 forms of similar bases, and inversely, horizontal edges of 

 combination may be considered as a certain character of 

 this property of forms, as is evident from the derivation of 

 the series itself, and of its limits. Ex. P co (/), 

 |P 1 (s\ P (o) and P 4- co (M ) in prismatic Topaz. 

 Vol. II. Fig. 34. 



ii. P + n. ( 4- n')< 



Let n' be = n ; and the forms accordingly co-ordinate 

 ones. The faces of P + n, meeting in their more obtuse 

 terminal edges, appear in the place of those terminal edges 

 of (P 4- n) m/ , which are situated contiguous to the prolonged 

 diagonal, and the edges of combination will be parallel 

 among themselves, and to the above mentioned edges of the 

 two pyramids. This parallelism follows from the simulta- 

 neous increment of the axis and of the variable diagonal of 

 (? + n)-' (. 94.). 



iii. P + n. (P + n') m '. 



Let n' be = n. Every thing is as in ii. ; except that the 

 faces of P 4- n meeting in the more acute terminal edges, 

 appear in the place of the similarly situated terminal edges 

 of (P 4- n) m/ . This likewise follows from . 94. Ex. P (P) 

 and (P) 3 (a) in prismatic Melane-glance. Vol. II. Fig. 34. 



iv. P 4- n. (Pr 4- n')< 



Let n' be = n ; m == 3. The combination will be 

 P + n. (Pr 4- n) 3 . The faces of P 4- n meeting in its 

 more obtuse terminal edges, are situated in the place 



