TERMINOLOGY. . 151. 



ever, likewise appear as rhombs in the combination, for the 

 edges between the two forms are parallel to the terminal 

 edges of P + 1- From the equation given in . 149., we 



have 



i o + i 



m = 2 + 1 = 2 + 1 = 3, and (P) = (P) 3 . 



The perfectly determined designation of the developed 

 compound form, is therefore 



P. P + 1. (P)3. P + co. [P + ]. 



a b c d c 



The indeterminate designation of the compound form, re- 

 presented in Fig. C8., is 



P + n. P + n 1 . (P + n") m . P + oo. [P + cc]. 

 a f c d e 



If we compare the present combination -with the pre- 

 ceding one, we find a perfect identity between the forms 

 P + n and P, and between the forms (P -I- n 11 )* 1 and (P) 3 , 

 the rectangular prisms likewise being common to both ; 

 and accordingly we may consider these forms as already 

 known, so that in 



P. P + n 1 . (P) 3 . P + oo. [P -f w]. 



a f e d e 



the only form still to be determined is the four-sided pyra- 

 mid P + n 1 (/). 



The faces of the scalene eight-sided pyramid (P) 3 , meet- 

 ing in their more obtuse terminal edges, are found in the 

 combination to appear in the place of the terminal edges of 

 P + n 1 . The two forms are therefore in a parallel position, 

 and the relation existing among them, if compared with 

 those considered above, is comprised under the case . 149. 

 ii.7- 



If the number 3 be substituted for m, the axis of the 



