LAWS OF DECREMENT. 



343 



TJTE RIGHT RHOMBIC PRISM. 



Its elements. 



The inclination of terminal to lateral planes r= 90'. 

 Inclination of lateral planes to each other 

 varies in different minerals; call the greater 

 angle at which they meet /, . 

 Terminal plane angles being equal to the 

 angles of the prism, are consequently /,, 

 and (180 I t ). But being plane angles, we 

 shall designate the greater by A t , and the 

 lesser by A z . 



Half the greater diagonal of the terminal plane 

 : half the lesser diagonal :: tang. \ A l : R. 

 Lateral plane angles 90. 

 Terminal edges equal. 

 Lateral edges equal. 



Ratio of a lateral to a terminal edge is deter- 

 minable from secondary planes only. 



Its units of comparison. 

 Fig. 348. 



The unit of comparison for determining the dif- 

 ferent laws of decrement by which this class of pri- 

 mary forms may be affected, will be different for 

 nearly all the different classes of modifications. 



