and Differential Dorthle Refraction. 353 



made for me by Bernard Halle of Berlin. Through the 

 courtesy of President Kohlrausch I was enabled to make 

 the comparison of the crystalline plates at the Reichsanstalt 

 in Charlottenburg. 



Comparison of Right- and Left-handed Quartz. 



Previous direct measurements of e and w in right- and 

 left-handed quartz indicate slight discrepancies, probably 

 instrumental errors. This method of comparison gives a 

 very sensitive test as to the interesting question of differential 

 double refraction in the two kinds of crystals. Accordingly, 

 wedges cut with their lengths parallel to the optic axis were 

 placed before S and S', and the latter illuminated with sodium 

 light, so that the path of the ray was normal to the optic 

 axis within the crystal, and adjustment made for coincidence 

 of the cross-hair and the sodium line in the two spectra. 

 This gave perfect coincidence throughout the spectrum, ad- 

 justment being made so as to bring the cross-hair on one of 

 the black bands, about ten bands being visible in the field. 

 One of the micrometer-screws was then turned so as to 

 increase the thickness of the wedge, and the number of bands 

 passing the cross-hair toward the red was counted until 

 perfect coincidence of every alternate band in this spectrum 

 with the bands of the former was obtained. The number of 

 black bands counted — namely, twenty — gave the original 

 order for the sodium line. The second wedge was then shifted 

 in the same way until perfect coincidence was obtained, the 

 number o passages across the sodium line being the same as 

 in the first case, namely twenty. 



This comparison was extended to the fifty-eighth order for 

 sodium (as high as available with the wedges at hand) and 

 not the slightest deviation from perfect coincidence through- 

 out the spectrum could be observed. The variation of one 

 passage would destroy the coincidence in a marked degree, 

 so that this method of comparison gives a simple and positive 

 method of determining the order. Different ratios of coinci- 

 dences, as 2 : 1, 3:1, 4:1, 3:2, 1:2, 2:3,... were 

 used, and the same orders were always obtained when sub- 

 stituted in equation (15). It may be concluded, therefore, 

 that the differential double refraction in quartz is the same 

 whether it be right- or left-handed. 



Comparison of Selenite and Left-handed Quartz. 



In the comparison of selenite and quartz the previous 

 wedge of left-handed quartz was used, and also a wedge of 

 fhil Mag. S. 5, Vol. 48. No, 293. Oct. 1899, 2 C 



