34 



object of the orthographic method. The disadvantage of the 

 method is that no angles and only those edges which are parallel 

 to the c axis are given their true size in the drawing ; one method 



therefore supplements 

 the other and crystals 

 should be illustrated 

 by both methods. 



In the clinographic 

 method the crystal is 

 projected upon a ver- 

 tical plane containing 

 the c axis. The c axis 

 is the only one given 

 its true length in pro- 

 jection. In Fig. 49 

 an octahedron is rep- 

 resented in ortho- 

 graphic projection; in 

 order to pass to the 

 clinographic two steps 

 are necessary : 1st, the 

 crystal is revolved 

 around the c axis some 

 selected angle, usually 

 18 26', after this revo- 

 lution the octahedron 

 will assume the posi- 

 tion of the dotted 

 lines. The axis oa will be moved to oa' ; when projected upon the 

 plane of projection of which xy is the trace it will appear foreshort- 

 ened as oa" ; in the same manner oai will appear upon the plane 

 of projection as oa/ ; the amount of foreshortening will depend 

 upon the angle of revolution. 2d, after the revolution about the 

 c axis, in order that the plane of the lateral axes shall not be repre- 

 sented in the drawing, by a line, as xy, the eye, hitherto at infinity 

 in a horizontal direction, is now elevated until the lines of vision 

 form an angle, selected generally as 9 28', Fig. 50, with the horizontal 

 plane. It is as if the eye, being at the same height above the floor 

 as the table top, the relative positions of objects on the table 

 would not be appreciated, as the lines of vision are parallel to the 

 table top ; if the eye is elevated, the top will come immediately into 



FIG. 49. 



