292 Wright — Optical Character of Birefr acting Minerals. 



An examination, of the curves for the several optic axial 

 angles, figs. 3 and I, indicates clearly that the convex side of 

 the bar in the diagonal position points toward the acute bisec- 

 trix. The determination of the optical character is effected 

 then most readily by means of the reel of the first order plate. 

 If the achromatic bar of the interference figure be placed in 

 the position of fig. 3 with the convex side of the hyperbola 

 pointing to the northeast and the arrow of the plate (direction 

 of least ellipsoidal axis) also in the same direction, the convex 

 side of the curve will show a blue interference color if the 

 mineral is optically negative ; the blue spot will be on the con- 

 cave side of the curve if it is optically positive. 



This method can always be applied if the convexity of the 

 curve can be discerned. In certain plagioclase feldspars, the 

 limiting case of 2V=90° is encountered occasionally and there 

 the bar is in fact a straight line. For most of the feldspars, 

 however, the curvature is sufficiently marked to enable a deter- 

 mination of their optical character. The result can be checked 

 by extinction angles of the section and adjacent twinning 

 lamellae after the method of Michel Levy. 



The approximate formula for the achromatic curves from this 

 plate can be derived from a discussion of fig. 5, which is based on 

 assumptions similar to those obtaining for the curves of the plate 

 perpendicular to the acute bisectrix. 



Fig. 5. 



The similar triangles AjPC and A 2 FE give at once the relations 



x _ x l — 2x 



(i) 



xy x .= yx 1 



\xy 



(2) 



