8 DEFINITIONS. 



An octahedron with a square base, fig. 14, contained 

 within eight equal isosceles triangular planes ; the 

 bases of the triangles constitute the edges of the base 

 of the octahedron. 



When the plane angle at a measures less than 60% 

 the octahedron is called acute. 



When the angle at a is greater than 60 , the octa- 

 hedron is called obtuse. 



The square base serves to distinguish this class from 

 the two which follow, it. The isosceles triangular 

 planes distinguish it from the regular octahedron. 



Fig. 15. 



d 



An octahedron with a rectangular base, fig. 15. The 

 planes of which are generally isosceles triangles, but 

 not equal. The plane angles at c and d of the planes 

 a and a' being more obtuse than those of the planes b 

 and b' '; and the planes a, and a', inclining to each 

 other at a different angle from that at which those 

 marked b] and b', meet. 



