200 TERMINOLOGY. . 13. 



those forms, of which only half the number of faces appear, 

 receive the additional divisor 2 ; and that those faces which 

 are turned towards the observer, contiguous to the upper 

 apex, are provided with the sign -f, while those contigu- 

 ous to the same apex, but on the opposite side, are distin- 

 guished by the sign . 



Only a few observations shall be made in the present 

 place on these combinations, in order to explain their ge- 

 tferal appearance. 



If the inclination of the axis be supposed = 0, the com- 

 bination of P 03 with P + OD, or with (P + cc) m , 

 (Pr + so) 3 , &c. will be a right oblique-angular four-sided 

 prism, which is a compound form, bearing altogether the 

 character of prismatic combinations, considered above. 



But if the axis be inclined in the plane of the greater 

 diagonal, or ,if the inclined face of the horizontal prism 



_? terminate the prism P -)- co ; then the combination 

 2 



will assume the appearance of an oblique-angular four-sided 

 prism, the basis of which is inclined to its acute lateral 

 edges. A similar prism is produced by a combination of 



P co or of r+n with an oblique-angularfour-sided prism, 

 only that the oblique terminal face is inclined towards the 

 obtuse lateral edges of the prism. If in ?^!? or ^ r + n 



n becomes = 4- co ; the horizontal prism is transformed 

 into a pair of planes parallel to the axis of the four-sid- 

 ed prism, and these faces appear with parallel edges of 



combination, in the case of ? instead of its acute la- 

 teral edges, in the case of l - instead of its obtuse ones, 



the prism itself remaining unlimited in the direction of its 

 axis. Combinations of this kind cannot be distinguished 

 from the prismatic ones P -f co. Pr + co, and P 4- cc. 

 Pr + co, unless some other faces be present, which, by their 



