56 jS. L. Penfield — Crystal Drawing. 



the line representing its linear projection. It is well to note 

 that the intersections x, y and z and .'#', y' and z' are in vertical 

 alignment with one another. 



Concerning the drawing of figure 25, it is a simple matter 

 to proportion the general outline of the barite crystal in ortho- 

 graphic projection. The direction of the edge between d, 102, 

 and o, Oil, is determined by finding the point a?, where the 

 lines of the linear projection of d and o intersect, and drawing 

 the edge parallel to the direction from x to the center c. The 

 intersection of the prism m, 110, with <^and o is a straight line, 

 parallel to the direction a to b or y to z. To construct the 

 clinographic figure, at some convenient point beneath the axes 

 the horizontal middle edges of the crystal may be drawn par- 

 allel to the a and b axes, their lengths and intersections being 

 determined by carrying down perpendiculars from the ortho- 

 graphic projection above. The intersection between d, 102, 

 and o, Oil, is determined by finding the point x' of the linear 

 projection and drawing the edge parallel to the direction from 

 x' to 1 {unity) on the vertical axis, while the corresponding 

 direction below is parallel to the direction x' to — 1. The size 

 of the prism m, 110, and its intersections with d and o may all 

 be determined by carrying down perpendiculars from the 

 orthographic projection above, but it is well to control the 

 directions by means of the linear projection : The edges be- 

 tween m, 110, and d, 102; and m, 110, and <9, 011, are parallel 

 respectively to the directions y' to 1 and z' to 1. Having com- 

 pleted a figure, a copy free from construction lines may be 

 had by placing the drawing over a clean sheet of paper and 

 puncturing the intersections of all edges with a needle-point : 

 An accurate tracing may then be made on the lower paper. 



Should it happen that the linear projection made on the 

 plane of the a and b axes gives intersections far removed from 

 the center of the figure, a linear projection may be made on 

 the clinographic axes either on the plane of the a and c or 

 b and c axes, supposing that the faces pass, respectively, through 

 unity on the b or the a axes. 



Importance of an Orthographic in connection with a Clino- 

 graphic Projection. — There is no question in the writer's mind 

 that many students, on commencing the study of crystallogra- 

 phy, fail to derive the benefit they should from the figures 

 given in text-books. Generally clinographic projections are 

 given almost exclusively, with perhaps occasional basal or 

 orthographic projections, and beginners find it hard to recon- 

 cile many of the figures with the appearance of the models 

 and crystals which they are intended to represent. For exam- 

 ple, given only the clinographic projection of barite, figure 24, 

 it takes considerable training and knowledge of the projection 



