38 CRYSTALLOGRAPHY. 
pair—that is, one of these planes and its opposite-—is called a 
hemiprism. 
In the rhombic prism, fig. 12, the four lateral planes are 
sunilar planes. But of the four lateral edges of the prism two 

are obtuse and two acute. Fig. 13 represents a combination of 
the rectangular and rhombic prisms, and illustrates the rela- 
tions of their planes. Other rhombic prisms parallel to the 
vertical axis occur, differing in interfacial angles, that is, in the 
ratio of the lateral axes. 
Besides vertical rhombic prisms, there are also horizontal 
prisms parallel to each lateral axis,a@and 0. In fig. 2 the narrow 
planes in front (lettered 47) are planes of a rhombic prism parallel 
to the longer of the lateral axes, and those to the right (17) are 
planes of another parallel to the shorter lateral axis. In fig. 6 
the planes are those of these two horizontal prisms. Such 
prisms are called also domes, because they have the form of the 
roof of a house (domus in Latin meaning house). In fig. 3 
these same two domes occur, and also the planes (lettered Z ) of 
a vertical rhombic prism. Of these domes there may be many 
both in the macrodiagonal and the brachydiagonal series, differing 
in angle (or in ratio of the two intersected axes). Those par- 
allel to the longer lateral axis, or the macrodiagonal, are called 
macrodomes ; and those parallel to the shorter, or brachydiag- 
onal, are called brachydomes. 
A rhombic octahedron, lettered 1, is shown in fig. 8; a com- 
bination of two, lettered 1 and 4, in tig. 9; and a combination 
of four, lettered 1, $, 4, 4, in fig. 10. This last figure contains 
also the planes J, or those of a vertical rhombic prism; the 
planes 1-2, or those of a dome parallel to the longer lateral axis ; 
the planes 1-2, or those of a dome parallel to the shorter lateral 
axis; the plane O, or the basal plane; the plane 7-2, or the 
brachypinacoid ; and also a rhombic octahedron lettered 1-3. 
2. Positions of Planes. Lettering of Crys‘als,—The notation 
is, in a general way, like that of the dimetric system, but with differ- 
ences made necessary by the inequality of the lateral axes. The letters 
