of Minerals in the Thin Section. 357 



With the universal stage, it is thus possible to measure the 

 optic axial angle of any grain of any transparent birefracting 

 substance and to distinguish the biaxial and uniaxial minerals. 

 The degree of accuracy of this measurement, however, is not 

 of the same order of magnitude for all sections, but differs 

 very materially with different sections. As a matter of experi- 

 ence it has been found that the most accurate results can be 

 obtained on sections in which both optic axes appear within 

 the field of vision ; that good results can be had from sections 

 which show only one optic axis within the field, while for 

 sections in which neither optic axis appears within the field the 

 determination is uncertain and at best only a rough approxima- 

 tion. 



To summarize briefly the different methods best applicable 

 to the four different possible cases cited above : 



(1) The optic axes are both within the field of vision and 

 inclined between 15°-55° with the normal to the section. 

 Determine approximate position of the two optic axes by bring- 

 ing each one, by means of H 2 and V 15 into the vertical position. 



Determine position of each more accurately by means of 

 optical curves in projection and check by means of extinction 

 curves and exact location of principal planes of ellipsoid, 

 especially the plane containing the optic axes. 



(2) Section is nearly normal to an optic axis ; one optic axis 

 A x inclined less than 20° to section normal. Place stage in 

 horizontal position, — H 2 and H 3 in horizontal position and Vj 

 normal to V 2 — turn H 3 and incline about V 2 until optic axis 

 coincides with the axis of the microscope ; then revolve about 

 Vj and turn H 2 until darkness is attainedj and thus determine 

 plane of optic axes and /3'. Incline V 2 back to 0° position, 

 revolve about H 2 until the optic axis coincides with plane nor- 

 mal to Vj and determine extinction curve, the intersection of 

 which with plane of optic axes in projection fixes the position 

 of the second optic axis accurately. Check by determining a 

 and 7 both from projection and observation ; also by extinction 

 curve for revolution about V 2 . 



(3) One optic axis inclined between 20°-55° within the crys- 

 tal to the normal of the section, the second entirely out of the 

 field of vision. Determine visible optic axis by optical curves 

 and second optic axis by means of extinction curves, both about 

 Vj and V 2 . Verify results by determination of a, /3 and 7. 



(4) Both optic axes are entirely without the field of vision, 

 i. e., are inclined at an angle of more than 65° in air with nor- 

 mals to the section. In such instances the location of the optic 

 axes is accomplished by means of extinction angles alone and 

 the values obtained are not accurate, since a slight error of 1° 

 in the- determination of the extinction angle may affect the 



