TERMINOLOGY. . 15 k 



Tetarto-prismatic combinations consist, like the prisma- 

 tic and the hemi-prismatic combinations, of forms, Avhose 

 general relations to each other are those developed for the 

 prismatic system. 



The tetarto-prismatic combinations depend upon the 

 fundamental form, . 98., Fig. 42., whose axis is inclined 

 in a plane which is perpendicular to the base, and passes 

 through neither of its diagonals. 



The designation of tetarto-prismatic forms must dis- 

 tinguish all the faces of the pyramid contiguous to one and 



the same apex. Thus the face BAC being turned towards 



p 



the observer on his right hand, is noted r , BAG' to his 



4 

 p 



left is noted 1 _ ; on the opposite side the face B'AC turn- 



p 



ed to his right hand is distinguished by r , the face 



4 



B'AC' turned to the left by 1 _ . In the same manner 



also the front and back faces of the horizontal prisms be- 

 longing to the diagonal BB', and the right and left faces 

 of the horizontal prisms belonging to the diagonal CC' are 



indicated by J-L+JL, and r _g r + n the latter of 

 2 2 



which extends also to those prisms whose axis is parallel 

 to the principal axis AA' of P. 



The tetarto-prismatic combination iLB. P + <x> gives 



4 



an oblique oblique-angular four-sided prism, in which the 

 alternating edges mutually are supplemental of each other. 

 Two of them assume a horizontal position, if the inclina- 

 tion of the axis is = 0. But if the angle of inclination is 

 an appreciable magnitude, or if these forms do not belong 

 to one and the same series, none of the edges of combina- 

 tion can become horizontal. 



The tetarto-prismatic combination . Pr + c . 



Pr -f os is an oblique four-sided prism, in which none of the 



