LAWS OF DECREMENT. 313 



REGULAR OCTAHEDRON. 

 Fig. 335. 





Its elements. 



The inclination of any two adjacent planes at their 

 common edge 109 28' 16", may be called /, . 

 Plane angles = 60, may be called A t . 

 Edges all equal. 



Inclination of edge to edge measured over the 

 solid angle 90. 



Inclination of plane to plane measured over 

 the solid angle = 70 31' 44", and may be 

 called J 2 . 

 Ratio of a perpendicular 



a b : an edge af : : ^3 i 2. 

 . . . an edge : | an axis :: ^2 : 1. 



Its unit of comparison 



Is 1, in reference to simple and mixed decrements 

 on the angles, and also to decrements on the edges. 



Simple and mixed decrements on the angles. 



The law of a simple or mixed decrement on the 

 angle of a regular octahedron may be determined by 

 means of the perpendiculars a b, a f, drawn from the 

 angle a upon the edges of the base : which perpen- 



