F. E. Wright — Measurement of Extinction Angles. 387 



. The final step in the adjustment is the alignment of the 

 crosshairs of the ocular with the principal planes of the nicols. 

 Many methods have been suggested for this purpose which 

 may be used to advantage. A mineral showing good cleavage 

 or lines of growth and parallel extinction (Hakes of anhydrite 

 or crystallites of quartz) serve well for the purpose. These are 

 first placed in the position of zero extinction (determined 

 accurately by means of bi-quartz wedge plate), and the cross- 

 hairs of the ocular brought to parallelism with the crystal edge 

 or cleavage line. The chief difficulty in this method lies in 

 the fact that it is exceedingly difficult to obtain suitable 

 material. 



E. Sommerfeldt* has recently suggested the use of a twinned 

 plate of selenite of sensitive tint. The plate is turned until 

 under crossed nicols the interference colors in the two halves 

 are of precisely the same tint, in which position the twinning 

 line parallels the principal planes of the nicols. This method 

 is sensitive and satisfactory, especially if, instead of a plate, a 

 wedge of the material be used, with which the interference 

 color can be changed until that particular tint, for which the 

 observer's eye is most sensitive, covers the field. In this con- 

 nection care should be taken to select a selenite plate in which 

 the twinning line is perfectly straight. In the selenite plate, 

 however, the angle which the adjacent ellipsoidal axis in each 

 half makes with the twinning line is 37-J- 3 , a very large 

 angle and not so well adapted to show slight deviations from 

 the true position as a twin of smaller symmetrical angle. In 

 this connection experiments with plagioclase lamella? were 

 tried but abandoned, since it was found by experience to be 

 exceedingly difficult to procure suitable material for the pur- 

 pose ; a simpler method can, however, be used, which accom- 

 plishes the same purpose more readily. On a thin plate of 

 quartz, cut parallel to the principal axis, an edge surface, 

 making an angle of 4° — 6°. with the principal axis, is first 

 ground and polished. The plate is then cut in half at right 

 angles with the polished edge surface and the two halves placed. 

 with their polished edges side by side and cemented with Can- 

 ada balsam, thus producing an artificial twin of any angle which 

 may be selected as most suitable and best adapted for fixing 

 the crosshair in the ocular. The twin is finally ground thin 

 and polished either to the sensitive tint or a pale gray of the 

 first order or in wedge form.f Like the selenite twin plate, it 

 is placed on the stage of the microscope and revolved under 



* Zeitschr. fur wissenschaftliehe Mikroskopie, xxiv, 24-25, 1907. 



f Such a plate and also wedge were prepared for the writer by Voigt & 

 Hochgesang of Gottingen. and have proved satisfactory in every reject. 

 The twinned selenite plate of sensitive tint cost 5 marks ; the wedge (I— III 

 order interference colors), 21 marks. 



