jS. L. Penfield — Crystal Drawing. 



57 



employed to gain from the figure a correct idea of the propor- 

 tions of the crystal which it actually represents. This may be 

 shown by comparing figures 24 and 26, which represent the 

 same crystal, drawn one with the a, the other with the b axis 

 to the front. It is seen from figure 26 that the crystal is far 

 longer in the direction of the a axis than one would imagine 

 from inspection of only the clinographic projection of figure 

 24. The front or a axis is much foreshortened in clinographic 

 projection, consequently by the use of only this one kind of 

 projection there is a two-fold tendency to err ; on the one hand, 

 in drawing, one is inclined to represent those edges running 

 parallel to the a axis by lines which are considerably too long, 

 while, on the other hand, in studying figures there is a tendency 

 to regard them as representing crystals which are too much 

 compressed in the direction 

 of the a axis. By using 

 orthographic in connection 

 with clinographic projec- 

 tions these tendencies are 

 overcome. Having in mind 

 the proportions of a certain 

 crystal, or having at hand 

 a model, it is easy to con- 

 struct an orthographic pro- 

 jection in which the a 

 and b axes are represented 

 with their true propor- 

 tions ; then the construc- 

 tion of a clinographic pro- 

 jection of correct proportions follows as a comparatively sim- 

 ple matter. Without an orthographic projection it would 

 have been a difficult task to have constructed the clino- 

 graphic projection of figure 26 with the proportions of the 

 a and b axes the same as in figure 24, while with the 

 orthographic projection orientated as in figure 26 it was an 

 easy matter. Then again, given a model for study, say of 

 barite corresponding to figure 24, a student holding the model 

 properly orientated, over or near to the orthographic pro- 

 jection, and looking down on it from above, sees at once 

 the relations between the model and the figure : Prismatic 

 angles have their true value in the drawing, and the directions 

 and relative lengths of all of the edges appear to be the same 

 as on the model. From an orthographic projection alone, 

 however, one can gain no conception of the length of a crystal 

 in the direction of the vertical axis, nor of the steepness of its 

 terminal faces : A combination of two projections is needed, 

 and from two figures a proper conception of the development 



