S. L. Pen field — Crystal Drawing. 



43 



Engraved Axes. — For the purpose of facilitating crystal 

 drawing the writer has had the isometric and hexagonal axes 

 engraved, and impressions of them made on good quality of 

 drawing paper have been found very useful. To insure accu- 

 racy they were plotted on a large scale (the vertical axis 28 cm 

 in length) and they are shown very much reduced in figures 9 

 and 10. Each axis from the center is divided into thirds, and 

 generally the lengths marked 1, when taken as unity, will give 

 a figure of convenient size for drawing. In figure 9 an ortho- 

 graphic and a clinographic projection of a dodecahedron are 

 shown, and in figure 10 corresponding projections of a combi- 

 nation of prism m and pyramid p of apatite, c — 0*735. As is 

 evident from the figures, the upper axes are for orthographic, 



9 10 



Figs. 9 and 10. — Scheme of the engraved axes of the isometric and hex- 

 agonal systems, one-sixth natural size. 



the lower for clinographic projections. The sections of the 

 axes marked 2 and 3 are lengths most frequently needed in 

 the construction of complex figures. Printed on each sheet is 

 a scale which will be referred to as the scale of decimal parts. 

 Its length is equal to that of unity on both the vertical axis 

 and the axes for orthographic projection. As printed on the 

 original sheets the scale is divided into one hundred parts. 



Axes of the Tetragonal and Orthorhombic Systems. — For 

 drawing tetragonal and orthorhombic crystals the engraved 

 isometric axes may be used, after changing certain lengths. 

 The vertical axis for both systems is changed by taking the 

 desired length from the scale of decimal parts, referred to 

 in the previous paragraph. For an orthorhombic crystal the 

 length of the brachy, or a, axis is first laid off on the front-to- 



