Mr. E. J. Chapman on the Notation of Crystals. 323 



tering and for the notation of crystals, in comprising strictly 

 the separate conditions of accuracy, simplicity, brevity, and 

 capability of being verbally expressed. The proposed method 

 is based entirely upon the position and relations of planes to 

 iheir crystallographic axes, and has no dependence upon the 

 external conjiguration of any fundamental form. The various 

 planes are also denoted by a few individual letters, so chosen 

 as to assist the memory ; and not by the position of certain 

 signs placed before, above, or around, a single letter, as in 

 the system of Naumann. The advantages which accrue from 

 this arrangement will, I think, be seen as we proceed. 



All crystallographic planes may be included under the 

 terms of prismatic and pyramidal ; the latter comprising 

 closed forms*, and, with the exception of the basal planes 

 of the oblique systems, all inclined summit-planes. 



Prismatic planes I designate by consonants-, pyramidal 

 PLANES by vovoels'X. Prismatic planes may be either monaxial 

 (cutting one axis), or diaxial, and in one system triaxial. 

 Pyramidal planes may be either diaxial, triaxial, or tetraxial 

 — the latter, of course, only in the hexagonal system. 



Monaxial prismatic planes may be of three principal kinds, 

 according to the system of crystallization. In the monometric 

 or regular system they are of one kind, and are lettered P. 

 In the dimetric or pyramidal system, they are of two kinds: 

 so that the letter P (from irlva^, table, as in Naumann's 

 Basische Pinakoid) is retained to designate the basal planes, 

 or those which cut the vertical axis ; and the letter M (from 

 yi,ovo<i) is employed for the vertical planes. Finally, in the 

 other systems we have P for the basal, M for the front and 

 back vertical, and L for the lateral or side vertical planes, 

 which cut but one axis. 



Diaxial prismatic planes are of one kind in the dimetric, 

 hexagonal, trimetric (rhombic), and monoclinic (oblique rhom- 

 bic) systems; and, as such, are lettered D (from 81?, tiao). In 

 the triclinic system they are of two kinds, U and B ibis, two), 

 the respective positions of which will be shown further on. 

 Triaxial prismatic planes can be only met with in the hexa- 

 gonal system : they are lettered T. 



The following symbols are consequently sufficient to denote 



all kinds of prismatic planes: — P; M; L; D; B; and T; 



* The cube omitted. 



t This idea is derived from Haiiy's method of designating planes on pri- 

 mary edges by consonants, and those on angles by vowels j but it must be 

 borne in mind that many inclined summit-planes are by this method denoted 

 by consonants, for the two systems are fundamentally quite distinct. 



Y2 



