4A 



S. L. Penfield — Crystal Drawing. 



back axis of the orthographic projection above by means of 

 the scale of decimal parts, and is then projected down upon 

 the front-to-back axis below by means of a vertical line. Thus 

 with facility and accuracy the engraved isometric axes may be 

 modified to suit the requirements of any tetragonal or ortho- 

 rhombic crystal. 



11 



Fig. 11. — Protractor for plotting crystaliographic axes; one-third natural 



size. 



Projection of the Axes of the Monoclinic and Triclinic 

 Systems. — These axes are obtained from those of the isometric 

 system by giving the lines suitable inclinations, and varying 

 their lengths. Instead, however, of using the methods gener- 

 ally employed for inclining the axes, it occurred to the writer 



