Fig. 9. 



Mr. E. J. Chapman on the Notation of Crystals. 329 



Observations. — In this system, in accordance with the ge- 

 neral custom, the longer horizontal axis is considered unity. 

 The planes A^, A^, &c. of the rhombic prisms of Levy and Du- 

 fr^noy correspond therefore to the planes D^ of the present 

 notation, because they abut more immediately upon the shorter 

 axis. The planes g\ ^, &c., on the other hand, are equiva- 

 lent to Dot. a value following a letter, invariably refers to 

 the relative length of the shorter horizontal axis compared to 

 that of the longer one as unity. Nauraann's symbols are, for 



D-, 00 P«; and for Dm, ocP«. 



The monaxial planes M and L form the vertical faces of 

 rectangular prisms ; whilst the biaxial pyramidal planes, j?A 

 and ajE, produce rectangular octahedrons. 



Monoclinic System. 

 Monaxial Planes : — 

 Basal planes, P. 



Back and front prismatic planes, M. 

 Right and left prismatic planes, L. 



Diaxial Prismatic Planes : — 



Ordinary oblique rhombic prisms, 

 D, D-, Dot. 



' to' 



Diaxial Pyramidal Planes : — 



Upper inclined planes in front, or 



lower behind. A, ^A, mA. 

 Upper inclined planes behind, or lower in front, I, ^I, wil. 

 Inclined planes at the sides, E, -E, wiE. 



Triaxial Pyramidal Planes : — 



Upper frontal planes of oblique rhombic octahedrons, or 

 lower back planes of the same, O, ^O, mO. 



Upper back planes of oblique rhombic octahedrons, or lower 

 planes in front of the same, U, ;|jU, mU. 



Intermediate planes, xOx ; x\5x. 



Observations. — In this system the orthodiagonal or horizontal 

 axis is made unity ; for if the clinodiagonal be selected as such, 

 according to perhaps the more usual custom, the positions of 

 the planes D^ and Dm will be the reverse of what they are in 

 the preceding system. By making, however, in each system 

 that axis unity which passes from left to right, the crystals 

 being in position, those planes which abut the more imme- 

 diately upon the frontal axis will invariably have the symbol 

 Di. 



