38 



CRYSTALLOGRA PHY. 



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 

 similar olanes. But of the four lateral edges of the prism two 



11. 



f*^~ 



-Q 



^ 





1 

 e 





u 





u 



-'" 







~^ 



12, 



r 



i : 



18 



^b- 



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 b. In fig. 2 the narrow- 

 planes in front (lettered ^l) are planes of a rhombic prism parallel 

 to the longer of the lateral axes, and those to the right (H) 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 I) 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 J and those parallel to the shorter, or brachydiag- 

 onal, are called br achy domes. 



A rhombic octahedron, lettered 1, is shown in fig. 8 ; a com- 

 bination of two, lettered 1 and -^, in fig. 9 ; and a combination 

 of four, lettered 1, |-, j$, 4-, in fig. 10. This last figure contains 

 also the planes I, or those of a vertical rhombic prism; the 

 planes l-i, or those of a dome parallel to the longer lateral axis ; 

 the planes 1-S, or those of a dome parallel to the shorter lateral 

 axis ; the plane 0, or the basal plane ; the plane i-i, or the 

 brachypinacoid ; and also a rhombic octahedron lettered 1-3. 



2. Positions of Planes. Lettering of Crystals.— 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 letten 



