5U 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



a plate of right-handed quartz, likewise superimposed and cemented side 

 by side as indicated in the figure, and in such a way that the line of 

 total extinction in the first combination is the extension of the line of 



Fig. 69. 



zero extinction in the second. This wedge in sodium light gives at the 

 extreme end symmetrical extinction of about ± 10° (fig. 70), while at 

 the thin end it is ±1°. Fig. 69 gives a view of the bi-jpuartz wedge 

 showing relative positions of right- and left-handed wedges and under- 



Fig. 70. 



lying left- and right-handed quartz plates, all normal to the optic axis. 

 Fig. 70 gives the top view of the bi-quartz wedge plate. The position 

 of the dark line of zero rotation, or exact compensation, is indicated 

 at C. 



CS^illluminatinff and.other Apparatus. 



ago 



Monochromatic Illumination.* — F. C. Hansen four years 

 described a dry yellowish green (light-green, naphthol-yellow) light 

 filter which, interposed between a Nernst light and the reflecting mirror 

 of the Microscope, produced good illumination by means of which sharp 

 images at high magnifications could be observed. The same author 

 now recommends the use of this filter in combination with the mercurial 

 vapour lamp of Schott. This lamp gives out yellow, yellowish-green, 

 and blue rays, and by means of the yellow-green light filter, a clear 

 monochromatic illumination is obtained. 



* Zeitschr. wiss. Mikrosk., xxvi. (1910) pp. 525-6. 



