105 
the angles of crystals. 
that the ordinate in AC is in the opposite direction : that is 
{p \ 'd {P ’ — 9 ’ co-existent planes. These may be 
included in the formula (p; ±q; r). 
4. The right oblique-angled Prism ^ fig. 10. 
21. It is obvious that the opposite angles A and D of the 
base of this prism are similar in all respects ; and with any 
secondary plane formed on one of them, we must have a 
co-existent similar plane on the other. That is, we must have 
a second plane, when x and y are negative, as they were 
positive in the first. Hence {p\ q \ r) ( — p\ — q ; r) are co- 
existent planes ; and we may express them thus [±p ; ± r) 
it being understood in such symbols that the upper signs are 
taken together, and the lower together. 
5. The right rhombic Prism ^ fig. 10. 
22. Here, the opposite angles A, D are similar, and also 
the adjacent sides. Hence with a plane {p\q;r) we have 
co-existent planes ( — p\ — q'-> p\ r)[ — q; — p ; r). These 
may be included in the symbol {±p, ±_q ; r) the upper signs 
being taken together as before, and p, q being permutable as 
is indicated by the comma. 
6 . The right rectangular Prism ^ fig. 11. 
23. Here the four angles A, B, C, D are similar. Hence 
; q \ r) has co-existent planes 
{—p-,q;r} {p; — q; r) r) 
These may be included in the formula 
the signs being taken in horizontal pairs. 
P 
MDCCCXXV. 
