LAWS OF DECRE3JEM. 571 



RHOMBOID. 



plane belonging to mod. e. The equation of this 



plane may be thus expressed, 



+ y. + 1 = i. 



p' ^ pf r 



From the character of the planes of mod. e, the 

 index p' is always =z 2 r. 



Knowing the relation of p' tor, we may discover 

 the relations of/? and q to r, by finding their relations 

 to p' ; and these relations may be known from the 

 equations of the traces of the two planes on the plane 

 of the x y, when referred to the points/I 



The equations of these traces are obtained by 

 making s = o in the two preceding equations, whence 



the equation of the trace d e, is -\- &- 1 (1) 



P 9 



and of b c ....... + = 1 (2) 



pf pf 



But as both traces pass through the point /^ tfce 

 values ot\r and of^y must be equal in both equations. 



Hence from equation (1), y q ^ ' r 



P 

 (2), y = pf jr 



Therefore . . q *LE = j/ x 



P 



x = 



1 



j ? 



3 A ^? 



