ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



513 



that of the obtuse bisectrix are indicated. Fig. 67 is similar to fig. 65, 

 except that the optic axial angle is 2 i> = 90 o . The dotted curves again 

 represent the directions for which the birefringence is 10 p.c. greater or 

 less than that of the bisectrix at the centre of the projection plate. 



Fig. 67. 



Plate b (figs. 59 and 68) is for the measurement of the optic axial 

 angle, and is simply a thin glass plate 1 " 5 mm. wide, on which fine co- 

 ordinate lines ■ 1 'mm. apart have been ruled. By means of this plate 

 the optic axial angle of a mineral can be measured, provided one or both 

 optic axes appear within the field of vision. 



±. 



mnij i i i in i i i iin 



*t 



t 



mm^ 



Fig. 68. 



Plate c (figs. 59, 69, 70) is a bi-quartz wedge plate for the accurate 

 determination of extinction angles. The dimensions are : — Plate of right- 

 handed quartz : length 35*3 mm., width 6 mm., thickness at thin end 

 35 mm., at thick end ■ 85 mm. ; plate of left-handed quartz : length 

 35*3 mm., width 6 mm., thickness 0*4 mm. The thin plate is 

 cemented on the wedge to a combination plate wedge which gives zero 

 extinction at a distance 3*5 mm. from the thin end (fig. 70). The 

 same specifications are followed with a wedge of left-handed quartz and 

 Aug. 17th, 1910 2 M 



