11 DEFINITIONS. 



angles similar, may be said to hare a similar axis 

 in three directions. 



But the lines c d, e f, joining the opposite lateral 

 solid angles of irregular octahedrons, may be called 

 the diagonals of their base. 



Fig. 29. 



The rhombic dodecahedron has two dissimilar sets of 

 axes passing through its centre ; one set, as a b, fig, 29, 

 passes through the pairs of opposite solid angles, 

 which consist each of four acute plane angles, and 

 may be called the greater axes ; another set, as c d, 

 passes through the solid angles which consist of three 

 obtuse plane angles each, and may be called the lesser 

 axes of the crystal. 



Fig. SO. 



Fig. 31. 



The right square, and right rectangular prisms, 

 have each an axis in four directions similar to a b, 

 fig. 30 and 31, but as prisms they have an additional 

 prismatic axis, c d. 



