SYMMETRY OF SECONDARY PLANES. 



Fig. 74. Fig. 75. 



Fig. 76, the same, with its obtuse laleral solid angles replaced by 

 tangent planes. 



Fig. 77, the same, with its terminal solid angles replaced by four 

 planes, reposing upon the primary planes. 



Fig. 76. 



Fig. 77. 



In the varieties of the quadrangular prism, the lateral edges hold 

 a different position from the terminal edges ; but sometimes the lat- 

 eral edges are all similar, at others, only two and two. The same 

 holds with respect to the terminal edges. * The solid angles may 

 also be either all identical in position, or they may occupy two dif- 

 ferent positions. 



In the right square Prism, the four lateral edges are~similar ; the 

 four terminal edges are so also, as well as the solid angles. The 

 modifications correspond to this identity, as may be judged of from 

 the following examples. 



