Wright — Optical Character of Birefr acting Minerals. 295 



optic axes respectively. The formula indicates clearly that, 

 except in the limiting case of 2Y=90°, the birefringence for 

 sections in the alternate quadrants containing the acute bisec- 

 trix is less than that for corresponding sections in the two 

 remaining quadrants. The rule resulting from this fact is that 

 the interference colors for points in the quadrants containing 

 the acute bisectrix are lower than those for corresponding 

 points in the direction of the obtuse bisectrix. 



After the direction of the acute bisectrix has been found by 

 one of the above methods, its value (c or a) can be readily 

 ascertained by ordinary methods either in parallel or conver- 

 gent polarized light. 



Summary. 



In the practical determination of minerals under the micro- 

 scope advantage is taken chiefly of those properties which are 

 definite in character and which can be readily ascertained. Of 

 these the optical character is one of the most useful since it 

 applies to all birefracting minerals and can be determined in 

 convergent polarized light on plates cut along one of several 

 different directions : 



1. On plates perpendicular to the acute bisectrix, by observ- 

 ing the direction of movement of the curves of the interfer- 

 ence figure on the insertion of a quartz wedge, mica plate, or 

 plate showing the interference color red of the first order. If 

 the loci of the optic axes lie outside of the field, determine 

 whether the plate is perpendicular to the obtuse or acute bisec- 

 trix by measuring the optic axial angle in air by the modifica- 

 tion of the Michel Levy method described on page 288. The 

 reduction of the observed optic axial angle to that in the crys- 

 tal can be accomplished only when the medium refractive index 

 of the substance is known and then easily by fig. 1. 



2. On a plate perpendicular to an optic axis by noting that, 

 when the black achromatic bar lies in a position diagonal to 

 that of the principal planes of the nicols, its convex side points 

 toward the acute bisectrix and that on the insertion of a plate 

 showing the interference color red of the first order, the con- 

 vex side will be colored blue if the arrow (n) of the inserted 

 plate lies in the plane of the optic axes and the mineral is 

 optically negative ; if the blue spot lies on the concave side of 

 the bar and the arrow of the plate still lids in the plane of the 

 optic axes, the mineral is optically positive. This method is 

 applicable whenever the curvature of the achromatic bar can 

 be observed. The section is moreover easy to find because in 

 parallel polarized light with nicols crossed it remains nearly 

 dark for all positions of the stage. 



