378 F. K Wright — Measurement of Extinction Angles. 



by the parallelism of its ellipsoidal axis with the principal 

 planes of the nicols. So soon as the crystal is turned even a 

 very small angle out of this position, the intensity of illumina- 

 tion of the two fields is no longer equal. By inserting or 

 withdrawing the combination wedge, the most advantageous 

 angle of rotation in the two fields can be procured so" that 

 the difference in intensity between the two halves is most 

 apparent. In effect this wedge is identical with that of the 

 bi-nicol ocular described above, is much simpler in con- 



Fig. 12. 



Fig. 12. Bi-quartz wedge-plate. In the plate-wedge ground for the writer 

 the two quartz plates are 35 mm long, 6 mm wide, and -35 mm thick. The 

 wedges are - 3 mm thick at the one end and # 5 mm at the thick edge. Cement- 

 ing material is Canada balsam whose refractive index is 1 '54, while « for 

 quartz is 1.544, a difference so slight as to render inappreciable the exceed- 

 ingly slight deviation of the waves caused by the slight wedge surface of the 

 wedge. This inclined surface is mounted next the Canada balsam and care 

 is taken by inserting a thin glass strip at the thin end to make the upper 

 and under surfaces of the completed wedge parallel. The thickness of the 

 wedge is '9 mn \ At the one end the rotation is ±1'1° ; at the thick end, ±3"2° 

 for sodium light, while at -85 n,m from the thin edge the rotation is zero in 

 both halves. For a wedge-plate of an angle of rotation 0° to 10° the follow- 

 ing specifications are suitable: length 50 llin \ width of each half 6 mm , total width 

 of wedge 12 mm . ; thickness of plate, "4 mni ; thickness of wedge at thin end 

 "35 mm ; at thick end, , 85 min . In such a wedge the point of zero rotation is 5 mm 

 from the thin end. At the thin end the rotation is ±1*1° ; at the thick end. 

 ±9"9°. In the article following this, specifications for a wedge with rotation 

 of 15° at the thick end are given. In preparing the wedge it is necessary 

 that the edges be ground and polished in order that the central division line 

 (fig. 11) be as sharp as passible. The two halves are eventually cemented 

 side by side with Canada balsam and any disturbing -influence thus eliminated 

 which might arise from total reflexion on the sides. 



struction, and requires no adjustment ; the one condition which 

 must be fulfilled for satisfactory results is that the wedge 

 be not tilted on insertion but that the optic axis remain always 

 parallel with the optic axis of the microscope, otherwise dis- 

 turbing birefringence phenomena appear. The wedge carriage 

 should, therefore, slide in an accurately fitting holder such as 



