372 APPENDIX CALCULATION OF THE 



RHOMBOID. 



In the like manner we may find 



1 

 But at the point/, x y, 



therefore . . . f^^J. = P f ~ P 



1-2. i -P 

 p 1 



whence . . . ' = ^ 1 



p+q 



q = 



2pp' 



As we know that p' 2 r, and that r cannot be 

 less than 1, p' cannot be less than 2 ; and it must, in 

 relation to any particular plane of mod. e, be either 

 2, 4, 6, 8, or some greater even number, according 

 to the number of molecules supposed to be contained 

 in the defect occasioned by that plane. 



It may be easily seen that when p' z= 2, we must 



have 

 2 



But as the indices of planes produced by inter- 

 mediary decrements must be whole numbers, it fol- 

 lows that the planes a b c, and a d c, cannot both 

 pass through the point f, unless p' be greater than 2. 



Let p' 4. and 4 = 



whence z= * 

 ? -2 



If we regard the figure 360, we may perceive that 

 if o b, which we have called/?', be considered equal 



