i RY8TALLOGRAPHY 



FIG. 48 a. 



remain.- the dome, d (102) : a c.n-t ruction section of the crystal, 

 containing the & and c axes, nm-t lie drawn, l-'ig. 48 a ; from b 

 draw be at right angles to the lia-e. Make be = b (the unit on b) 

 X 1.313, and ba = b X .815, connect c and a which will repi 

 the ridge of the unit pyramid p (in). 

 The base c cuts this pyramid at a 

 point p, found by laying off bo equal 

 to cc', l-'ig. 48, projecting up to the 

 ridge of the pyramid at p, then pc 

 will be the base at the same height at 

 which it is represented in Fig. 48. 

 The form (102) will cut the a axis at 

 a', ba' = b X .815 X 2, and the c axis 

 at 6, be = b X 1.313 ; ca' connecting 

 these two points will give the slope 

 and angle of the macrodome (102), 

 this may be moved in parallel to ca', to cut the pyramid at any 

 required point as at cd; project cd to cd' and lay off on the a 

 axis in Fig. 48 from c, ex, and cy, also ex' and cy' = bd' and be', 

 then yy' will be the projections of the points at which the dome 

 enters the edge of the pyramid; through xx' draw lines parallel to 

 the b axis, which will be the intersection of the dome with the 

 base ; then the triangle xzy, and similarly above x'z'y', will repre- 

 sent the dome faces. 



Uniformity of lettering. It is the custom to adopt a uniform 

 system of lettering crystal forms, at least those forms which deter- 

 mine the crystalline characters, that they may be recognized at 

 once and the positkm of the axes fixed. The pinacoid (100) = 

 a; (010) = b; (*B3) = c; m represents the unit prism (110); 

 p, the unit pyramid (111). 



In the hexagonal and tetragonal systems the prism of the first 

 order is m, that of the second order a, and the unit rhombohedron is 

 r. In the isometric system a, o, and d represent the cube, octahe- 

 dron, and rhombic dodecahedron respectively. In addition to the 

 above, individual faces are indicated by accents ; all faces in the 

 right front octant are not accented, faces in the right back octant 

 are indicated with one accent, etc.; thus p, p', p", p'" would 

 indicate the four upper faces of the unit pyramid. 



Clinographic projection. The clinographic method of illustra- 

 tion, in addition to expressing the relation of various forms, also 

 gives the impression of solidity and perspective, which is % not the 



