DEFINITIONS. 



Fig. 26. 



In the cube, an axis passes through the centre and 

 through two opposite solid angles, a , fig. 26 ; from 

 the perfect symmetry of its form, the cube has a simi- 

 lar axis in four directions, or passing through its 

 centre and through each pair of opposite solid angles. 



Fig. 27. 



The axis of the regular tetrahedron passes through 

 the centres of the summit and base as a b, fig. 27, and 

 it has a similar axis in four directions in consequence 

 of the symmetrical nature of its form. 



Fig. 28. 



In all octahedrons the axis passes through the two 

 summits and through the centre of the base, as a b, 

 fig. 28; the regular octahedron, having all its solid 



