the angles oj crystals, 103 
-j q, -^=zr, (p ; q; r) may still be taken for the symbol 
of the plane. In this case are the co-efficients of 
the equation to the plane, and are to be used for A, B, C in 
calculating the angles which the planes make with each other. 
We shall use the following terms ; a rhombic prism is one 
whose base is a rhombus : an oblique rhombic prwn, fig, 8, is 
one in which the sides are not at right angles to the base, the 
angles of the sides, as BA %, CA z being equal. A doubly 
oblique prism ^ fig. 7, is one in which the angles of the sides 
at the base BA 2:, CA 2: are unequal. Prisms are called square 
or rectangular when their bases are so : and when the base is 
a parallelogram with unequal sides, and angles not right an- 
gles, the prism is called oblique-angled. Besides these we have 
a prism which we may call the oblique rectangular prism * 
fig. 9, in which besides the two rectangular ends we have 
two sides, cz and the opposite one, also rectangles. 
1. The doubly-oblique Prism, fig. 7. 
18. In this, since the angles are all different, no one of the 
solid angles (A, B, C, D) is similar to another. Hence if a 
plane be formed on one of the angles, there is no plane ne- 
cessarily formed on another angle ; consequently a plane as 
(/> ; g ; r) or (^ ; — q ; — r) does not necessarily imply any co- 
existent plane, and the symbol is to be written with the mark 
( ;) between the indices, to show that no permutations are 
allowed. 
Let the edges of the subtractive prisms in last article be a, 
* We might consider B z as the base of prism, by which means it would be a 
right oblique angled prism. But the method adopted in the text seems to be more 
natural and simple. 
