DECREMENTS. 





Fig. 98 shews the character of a similar plane on 

 the edge of a tetrahedron. 



Fig. 99. 



Fig. 99 shews the character of a similar plane on 

 the edge of an octahedron. 



If a simple or mixed decrement take place on the 

 angle of a tetrahedron or octahedron, one new edge 

 c' d', of the secondary plane, will be always parallel 

 to an edge a b, of the primary form, as shewn in 

 figures 98 and 99. 



If either a simple or mixed decrement take place on 

 the angle of any primary form, except the tetrahedron 

 and octahedron, a new plane is produced, whose in- 

 tersection with the primary plane, along which the 



