92 
Mr. Whewell on calculating 
APrsjc'^, AQ = x'^, AR = a"r, the plane PQR will be 
parallel to p qr, and its equation will be 
= orpx — qy — rz = i: 
and its symbol — -i-; ^j,or(/>; — q-, — r;). Hencea 
plane which cuts off the inferior solid angles is distinguished 
by having two negative indices. 
It may be observed, that in both these cases the coexistent 
planes are given by taking the permutations of p,q,r; and 
may be represented as before by ( — p, q, r) and [p, — q; — r). 
There will in each case be six ; two for each angle. 
5. If one of the quantities AP, AQ, AR, or h, k, /, in any 
of these cases become infinite, we shall have a truncation of 
an edge of the rhomboid. Thus if AP, in fig. 3, become in- 
finite, we have a plane cutting off the terminal edge A x, 
fig. 5. And since h is infinite, if g = - 1 , r = ^, the equation 
of this plane is qy + = 1 ; and its symbol (o ; q\ r). 
In the same manner, making x'r infinite in fig. 4., we have, 
for a plane truncating the lateral edge x'y, an equation 
px — qy zsii, and a symbol ( /> ; — q; o ) . 
The terminal edges of Ax, Ay, Az, are not similarly 
affected with the lateral edges xy\^ %, zx', x'y, yz\ z! x. 
6 . Instead of supposing the secondary faces to be produced 
by removing a part of the rhomboid A a, we may conceive, 
with Hauy, that this larger figure is composed by adding 
successive layers of the small component rhomboids to a 
rhomboidal nucleus ; and that the secondary faces are pro- 
duced by supposing the magnitude of these layers to de- 
crease according to any law. And it will be easy to show 
