of Minerals in the Thin Section. 



335 



the intersection of the great circle A/A/, and the diameter 

 OC, the trace of the plane of vibration of the lower nicol. 



Having thus determined the point A/ and C ; the projection 

 of the second optic axis A 2 is found by making A 2 ' C = A/ C 

 (Biot's law). The intersection of the great circle PA/ with 

 the plane of optic axes A t A 2 determines then the position of 

 A s , and the angle A^, in projection is 2V, the angle between 

 the optic axes. 



The actual time consumed in this operation is not great, and 

 the values obtained are approximately correct. The objection 



10 



Fig. 10. In this figure the operations of actual construction are given 

 which are required to measure A X A 2 from the data in the drawing. The 

 points Ai and P are first located accurately in the drawing, reduced to 

 angtilar values and plotted directly on tracing paper in stereographic pro- 

 jection ; the great circle Ai'A 2 ' polar to Pi, and the horizontal great circles 

 A, A 2 are then sketched ; the point A/ is the intersection of the great circle, 

 containing P and Ai with great circle A/A 2 ', while the line OC is the trace 

 of the plane of vibration of the lower nicol (polarizer) as it appears on the 

 drawing after the revolution of S0° or 45°; the angle MOF indicating directly 

 the angle of revolution. By definition A 2 'C is equal to A/C, and the inter- 

 section A 2 of the great circle PA 2 ' with the great circle AiA 2 determines A 2 . 

 the second optic binormal, the angle A X A 2 being the desired optic axial 

 angle. 



to its use lies chiefly in the subjective factor involved, namely, 

 the skill required in drawing accurately the phenomena 

 observed, and also in the nice adjustment of all parts of the 

 instrument. 



The location of the optic axis Aj is at all times more accu- 

 rate and trustworthy than that of P, owing to the indistinct- 

 ness and width of the black axial bars near the margin of the 

 field in consequence, chiefly, of elliptic polarization. 



