54: 



S. L. Penfield — Crystal Drawing. 



24 



direction of an edge made by the meeting of any two faces, 

 but the principle depends generally upon locating two points, 

 common to both faces, where they intersect certain axial planes. 

 A line through the points thus found gives the direction of 

 the edge. In general it will be found best to adopt some 

 system for determining the direction of crystal edges, and 

 to adhere to it rather strictly, and the writer has found the 

 method based upon the linear or Quenstedt projection most 

 useful. The projection is too well known' to crystallographers 

 to need discussion ; as far as it relates to crystal drawing, how- 

 ever, it will be treated briefly in order 

 to add to the completeness of the present 

 article. 



The principle upon which the projec- 

 tion is based is very simple : Every face 

 of a crystal {shifted if necessary, hut 

 without change of direction) is made to 

 intersect the vertical axis at UNITY, 

 and then its intersection with the hori- 

 zontal plane, or the plane of the a and 

 b axis is indicated by a line. When it 

 is desired to find the direction of an edge 

 made by the meeting of any two faces, 

 the lines representing the linear projec- 

 tion of the faces are first drawn, and 

 the point where they intersect is noted. 

 Thus a point common to both faces is 

 determined, which is located in the plane 

 of the a and b axes. A second point com- 

 mon to the two faces is unity on the ver- 

 tical axis, and a line from this point to 

 where the lines of the linear projection 

 intersect gives the desired direction. 

 A simple illustration, chosen from the orthorhombic system, 

 will serve to show how the linear projection may be employed 

 in drawing. The example is a combination of barite. such as 

 is shown in figure 24. The axial ratio of barite is as follows : 



a\b\c~ 0-8152 : 1: 1-3136 



The forms shown in the figure and the symbols are, base c (001), 

 prism m (110), brachydome o (Oil) and macrodome d (102). 



Figure 25 represents the details of construction of the 

 orthographic and clinographic projections shown in figure 24. 

 On the orthographic axes the axial lengths a and b are located, 

 the vertical axis c being foreshortened to a point at the center. 

 On the clinographic axes, centered at O, the ends of the axes 

 a and b are located by dropping perpendiculars from corre- 



