. 152. OF COMBINATIONS. 195 



n 



isosceles four-sided pyramid will be = 3. 2. 5 . a, or = 3. a if 

 n = 0. Hence the pyramid is that member of the first 

 subordinate series which belongs to P + 3 (. 149. ii. 4.), 

 that is to say, it is -f + 3. 



The complete designation of the compound form follows 

 according to this developement, thus : 



P. -^ P 4- 3. (P) 3 . P + co. [P + ao ]. 

 a fed e 



. 152. PRISMATIC COMBINATIONS. 



A combination of the prismatic system is said more 

 particularly to possess a Prismatic Character, if 

 the forms contained in it appear with the whole 

 number of their faces. 



The number of binary combinations in the prismatic 

 system is so great, on account of the great variety of differ- 

 ent relations among its forms, that it becomes impossible 

 in the present place to consider them all, even though this 

 should be done in the most general manner. We can 

 therefore notice only as many as will be sufficient for ex- 

 plaining the greater part of the cases commonly occurring 

 in nature, and which, at the same time, shew how to pro- 

 ceed in the application of the methods of developement 

 mentioned above (. 143.). These binary combinations are : 



