S. L. Penfield — Crystal Drawing. 



53 



the right-to-left of b axis, and its left-hand edge parallel to the 

 front-to-back or a axis of the orthographic projection. A sec- 

 ond triangle IT is shown in two positions in figure 23 ; Ila, 

 when its shorter edge is against the blade of the T-square its 

 right-hand edge is parallel to the vertical axis, and, II&, when 

 one of its longer edges is against the blade of the T-square 

 its upper edge is parallel to the front-to-back or a axis 



of the clinographic projection. A third triangle III, figure 

 23, is for the hexagonal system, and is so made that when its 

 longer edge is against the blade of the T-square its upper left- 

 hand edge is parallel to the a } , —a x axis, and its upper right- 

 hand edge parallel to the « 3 , — a 3 axis of the clinographic pro- 

 jection ; compare figure 10. Thus with T-square and triangles, 

 the axial directions, the essential ones in the construction of a 

 crystal figure, may be had almost instantly, excepting, of course, 

 some of the directions of the monoclinic and triclinic systems. 

 Uses of the Linear or Quendstedt Projection. — In drawing 

 crystals various methods may be employed for finding the 



