1886.] 



Light by Reflection from Iceland Spar. 



185 



from the natural substance, but that an artificial surface polished 

 with chalk behaves very nearly, if not exactly, like a natural one. 



Seebeck's measurements were all made with the crystal in air, and 

 as the changes in the azimuth of the plane of polarisation, and in 

 the value of the polarising angle, for different azimuths of the crystal, 

 when such is the case are small, it seemed desirable before making 

 any measurements with the artificial surfaces cut perpendicular and 

 parallel to the axis, to make some determinations with an artificial 

 surface parallel to a natural face of the crystal when the crystal was 

 immersed in water ; this was accordingly done. 



Contrary to the orders that had been given, the edges of the plate 

 were cut away by the optician who polished it, and it was therefore 

 impossible to determine the position of the principal section in the 

 same manner as that previously employed with the other crystals. 

 The results were therefore plotted, the divisions of the ring carrying 

 the crystal being taken as abscissas and the azimuths of the plane of 

 polarisation of the reflected light as ordinates. It was assumed that 

 the curve for the artificial face would be similar to that for the natural 

 face, and the points at which the curve cut the x axis were taken 

 as indicating the position of the principal section, and the azimuths 

 •of the crystal thus determined. Table VIII gives the results : — 



Table VIII. 



Artificial Surface of Iceland Spar in Water. 





















Difference 



Azimuth 



Azimuth 







Azimuth 



Azimuth 







in the values 



of the 

 principal sec- 

 tion of the 

 crystal. 



of the 

 plane of 

 polarisation. 



Angle of 

 polarisation. 



of the 

 principal sec- 

 tion of the 

 crystal. 



of the 

 plane of 

 polarisation. 



Angle of 

 polarisation. 



of the 

 polarising 

 angle at 

 and 





















+ 180°. 







o 





o 



185 



o 



57 



o 



/ 





5 



+ 32 



46 



47 



+ 



47 







-13 



15 



+ 1 37 



46 



35 



195 



+ 2 



42 



46 



40 



- 5 



25 



+ 2 12 



47 



37 



205 



+ 4 



55 



47 



12 



+ 25 



35 



+ 3 05 



47 



10 



215 



+ 6 



40 



47 



36 



-26 



45 



+ 3 



48 



48 



225 



+ 7 



25 



48 



26 



+ 22 



55 



+ 3 02 



48 



47 



235 



+ 8 



45 



48 



50 



- 3 



65 



+ 47 



50 



13 



245 



+ 9 



27 



49 



59 



+ 14 



75 



+ 02 



50 



11 



255 



+ 8 



55 



50 



06 



+ 5 



85 



-3 33 



51 



37 



265 



+ 7 



45 



51 



06 



+ 31 



95 



-4 13 



50 



38 



275 



+ 4 



10 



50 



46 



- 8 



105 



-6 38 



50 



26 



285 



+ 2 



17 



•50 



39 



-13 



115 



-7 20 



49 



44 



295 



+ 1 



22 



50 



37 



-53 



125 



-8 



49 



53 



305 



-0 



35 



50 



05 



-12 



135 



-7 10 



48 



37 



315 



-1 



28 



48 



55 



-18 



145 



-7 15 



48 



41 



325 



-2 



58 



48 



34 



+ 7 



155 



-4 38 



47 



41 



335 



-2 



08 



47 



07 



+ 34 



165 



-3 38 



47 



23 



345 



-1 



35 



47 



13 



+ 10 



175 



-1 30 



46 



21 



355 



-0 



08 



46 



02 



+ 19 



