630 J. W. EVANS ON THE DETERMINATION OF MINERALS. 



Fig. 9. Pendulum movement obtained by rotation from fig. 7. 

 ,, 10. Fan movement from fig. 7. 

 ,, 11. Parallel movement from fig. 7. 

 ,, 12. Fig. 7 rotated through 45. The lower colour in the 



north-east indicates the longitudinal direction and its 



proximal end. 

 ,, 13. Straight central isogyre of two branches meeting in a 



black cross in a biaxial crystal. 

 ,, 14. The same rotated through 45 with the vertices of the 



isogyre still visible in the field. The gypsum plate is 



inserted, and the lower colours on the concave sides 



of the curves show the crystal to be slow ( + ). 

 ,, 15. Fig. 13 rotated through 45 in a case in which the 



vertices of the isogyre have disappeared. The lower 



colours indicate the longitudinal direction. 

 ,, 16. Curved central isogyre with gypsum plate. The lower 



colours on the farther side of the isogyre indicate that 



the section is fast ( ). 

 ,, 17. Section at right angles to one of the optic axes of a 



biaxial mineral. The optic axial plane lies east and 



west. 

 ,, 18. The same rotated through 45 when the crystal is neutral. 

 ,, 19. The same when the crystal is not neutral. The gypsum 



plate is inserted, and the higher colours on the concave 



side of the curve show that the crystal is fast ( ). 

 ,, 20. The same with smaller optic axial angle showing two 



branches of the isogyre. 

 ,, 21. Section at right angles to the optic axis of a uniaxial 



crystal that is to say, one in which the acute optic 



axial angle is zero. The higher colours in the angles 



traversed by the slot and plate show that the crystal 



is fast ( ). 



The directions of the arrows in figures showing isogyres in- 

 dicate the relations between the angular directions of the move- 

 ments of the ends of the isogyre and that of the rotation of the 

 stage, and their lengths the relative angular velocities. 



Journ. Quekett Microscopical Club, Ser. 2, Vol. XII., No. 11, November 1915. 



