46 



S. L. Penfield — Crystal Drawing. 



12 



c\o.6'2l 



^3- 108' ^9' 



\ 



The projection of the a axis, which is the same for both the 

 monoclinic and trielinic systems, will first be explained : 

 "When a is not at right angles to c, it must appear somewhat 



foreshortened in orthographic pro- 

 jection, as shown in figure 12, 

 which represents the relations of 

 the a and c axes of rhodonite : 

 XY being the trace of the hori- 

 zontal plane on which the ortho- 

 graphic projection is made, the a 

 axis, length 1-073, will appear fore- 

 shortened to the length Oa' . 

 ! Applying the foregoing principle 



to the upper circle of the pro- 

 tractor, figure 13, draw a radius at the inclination /3, 108° 44', 

 making use of the graduation of the circle, lay off on this 



13 



aLY 



radius the length of the a axis (1*073 in figure 13) using the 

 scale of decimal parts, and then project at right angles to the 

 direction a, —a, as indicated by the arrow, thus determining 



