178 



Sir J. Conroy. On the Polarisation of [Feb. 4, 



the plane of incidence and the obtuse summit towards the side from 

 which the light was incident upon it, was therefore azimuth 180°. 

 The crystal was rotated clockwise, and the same direction of rotation 

 was considered the positive direction for the Nicol. 



Table I gives the measurements made in this way with a cleavage- 

 face of Iceland spar in air ; Tables II and III with the same face in 

 water and carbon tetrachloride. Table IV contains the measurements 

 made with another cleavage-face of the same crystal in water. 

 Tables V, VI, and VII, give the results with, a cleavage-face of a 

 second crystal in air, water, and carbon tetrachloride. 



Table I. 



Iceland Spar in Air. 



Azimuth 

 of the 

 principal sec- 

 tion of the 

 crystal. 



Azimuth 



of the 

 plane of 

 polarisation. 



Angle of 

 polarisation. 



Azimuth 

 of the 

 principal sec- 

 tion of the 

 crystal. 



Azimuth 



of the 

 plane of 

 polarisation. 



Angle of 

 polarisation. 



Difference 

 in the values 



of the 

 polarising 

 angle at 

 and 

 + 180°. 

















t 











/ 



+ 1 







+ 



10 



57 



20 



181 







+ 



22 



57 



06 



+ 14 



7 



20 



+ 



35 



57 



24 



187 



20 



+ 



47 



57 



09 



+ 15 



11 







+ 



30 



57 



17 



191 







+ 1 



10 



57 



12 



+ 5 



17 



20 



+ 



40 



57 



22 



197 



20 



+ 1 



50 



57 



16 



+ 6 



21 







+ 



25 



57 



24 



201 







+ 2 



05 



57 



25 



— l 



27 



20 



+ 



45 



58 







207 



20 



+ 2 



35 



57 



29 



+ 31 



31 







+ 



47 



57 



50 



211 







+ 3 



15 



57 



59 



— 9 



37 



20 



+ 



37 



58 



11 



217 



20 



+ 3 



25 



57 



59 



+ 12 



A 1 



A 

 U 



_i a 



+ u 



A a 



CO 

 DO 



1 >7 

 1/ 



991 



A 

 U 



+ O 



OA 



oU 



CO 

 OO 



^ a 



— 2 



47 



20 



+ 



47 



58 



39 



227 



20 



+ 3 



37 



58 



31 



+ 8 



51 







+ 



35 



58 



34 



231 







+ 4 



05 



58 



34 



o 



57 



20 



+ 



35 



59 



02 



237 



20 



+ 3 



42 



58 



52 



+ 10 



fa 







-0 



03 



58 



48 



241 







+ 3 



45 



59 



03 



— 15 



67 



20 



-0 



10 



59 



18 



247 



20 



+ 3 



32 



59 



12 



+ 6 



71 







+ 



02 



59 



13 



251 







+ 3 



37 



59 



15 



- 2 



77 



20 



-1 



08 



59 



32 



257 



20 



+ 3 



50 



59 



42 



-10 



81 







-1 



25 



59 



24 



261 







+ 3 



05 



59 



32 



- 6 



87 



20 



-1 



43 



59 



35 



267 



20 



+ 3 



10 



59 



20 



+ 15 



91 







-1 



33 



59 



49 



271 







+ 2 



45 



59 



38 



+ 11 



97 



20 



-2 



30 



59 



26 



277 



20 



+ 2 



12 



59 



33 



- 7 



101 







-2 



30 



59 



38 



281 







+ 2 



35 



59 



45 



— 7 



107 



20 



-2 



33 



59 



25 



287 



20 



+ 1 



15 



59 



23 



+ 2 



111 







-2 



23 



59 



11 



291 







+ 1 



47 



59 



20 



- 9 



117 



20 



-2 



48 



59 



06 



297 



20 



+ 



52 



59 



03 



+ 3 



121 







-2 



58 



59 



04 



301 







+ 



55 



58 



56 



+ 8 



127 



20 



-3 



13 



58 



43 



307 



20 



+ 



30 



58 



43 







131 







-3 



05 



58 



37 



311 







+ 



05 



58 



37 







137 



20 



-3 







58 



25 



317 



20 



-0 



20 



58 



25 







141 







-2 



55 



58 



10 



321 













58 



17 



- 7 



147 



20 



-2 



37 



57 



47 



327 



20 



+ 



07 



58 



06 



-19 



151 







-2 



20 



57 



54 



331 







-0 



03 



57 



48 



+ 6 



157 



20 



-2 



08 



57 



26 



337 



20 



-0 



08 



57 



54 



-28 



161 







-1 



33 



57 



43 



341 







-0 



25 



57 



30 



+ 13 



167 



20 



-0 



48 



57 



22 



347 



20 



-0 



06 



57 



22 







171 







-1 







57 



28 



351 







+ 



05 



57 



23 



+ 5 



177 



20 



+ 



12 



57 



14 



357 



20 



-0 



13 



57 



27 



-13 



