EXPLANATION OF PLATE 14. 



Figure 1 — An elevation illustrating the mode of making a stereographic projection. 



Figure 2a — The sketch of a section of a twinned augite. (The two lines indicating 

 the twinning should he parallel.) 



Figure 2 — A stereographic projection of the same section. SC^ is taken parallel to 

 the twinning line of 2a. 



Figure 3 — A stereographic projection of tourmaline, illustrating the section given in 

 the lower left-hand part of the figure. 



Figure 4a — An image in convergent light. 



Figure 4 — Diagram illustrating the determination of the position of the section 

 which gives that image with the aid of the stereographic projection. 

 It is located at S. 



Figure 5 — A scale to facilitate the use of the stereographic projection. The lower 

 scale is of long chords. 



Figure 6 — Diagram showing the curves, any one of which is the locus of face-points 

 of plane sections which cut a dihedral angle of 45°, so that the plane 

 angle traced in them has the same value as that appended to the 

 curve. 



Figure 7 — Diagram illustrating the connection which must exist between the ap- 

 parent angles of termination of tourmaline rhombohedra in cross- 

 section ; also observations upon the same. 



(382) 



