hy Mecuis of Graphical Methods. 



259 



that at the poles h aud c, figure S, where normals to the pina- 

 coicl 010 and the base 001 meet the sphere, the plane angles 

 /9, // and V and y, a and r of figure 6 are to be found. How 

 these facts are to be made use of, will subsequently be shown 

 by examples. 



Another principle which may be made use of for the solu- 

 tion of many problems is that based upon the conditions estab- 

 lished by three faces in a zone, known by some as the cotan- 

 gent and tano^ent relation." For the purpose of illustrating 

 this principle, reference is made to figure 10, which represents 

 a regular arrangement of dots conditioning angles like those 



10 



found in the prismatic zone of crystals of rhodonite. The 

 dots may be regarded as representing crystal particles or mole- 

 cules, and the forms shown by the heayy outliiie of the fio-ure 

 are the planes (/ (100), h (010), m (110), "^ilZ (110) and /(130), 

 as seen in orthographic projection. It may be assumed that 

 the crystal is made up of layer upon layer of particles like the 

 one shown, figure 10 being an orthographic projection of such 

 a system of particles, as seen in the direction of the yertical 

 axis of the crystal. Since rhodonite is triclinic, a single layer 

 of particles, represented by the dots, does not lie in a plane 

 parallel to the paper, but in a plane tilled down to the front 

 and right by an amount indicated by the axial inclinations 



*Dana's Text Book of Mineralogy, edition of 1898, p. 31. 



