LAWS OF DECREMENT. 355 



THE DOUBLY OBLIQUE PRISM. 



Its elements. 



The inclination of the primary planes is unequal at 

 an?/ three adjacent edges, and is different in 

 different minerals. 



Three adjacent plane angles unequal. 

 Three adjacent edges unequal, and the ratios 

 of these inequalities are to be deduced only 

 from some secondary planes. 



The laws of decrement which produce the modify- 

 ing planes of this class of primary forms, may be 

 determined by the general methods already described 

 at p. 295, and the units of comparison will then be 

 the ratios of the primary edges. 



