'Reflexion of Light from Iceland Spar. 



249 



some very fine emery was scattered over a tub of water and allowed 

 to settle ; after standing for a number of minutes, the liquid was 

 poured off, and the sediment, which was deposited on standiug for a 

 further number of minutes, was preserved for use. Five kinds of 

 emery were used, viz. : — 



Stood 1 minute, deposited in 5 minutes. 

 » 5 „ 15 



„ 15 „ 30 „ 



„ 30 „ 2J hours. 



„ 2 hours „ 40 „ 



The crystal was polished about twenty minutes with each kind in 

 a perfectly quiet room to avoid dust being deposited on the glass and 

 causing flecks in the crystal. 



Next a bed of refined pitch was prepared having its upper surface 

 perfectly plane. Upon this the crystal was polished with rouge for 

 about three hours. Finally, the surface was carefully cleaned by 

 washing it in a stream of water. 



VII. Experiments made with the same Face Polished. 

 The screens were carefully set in their places, and by means of 

 these the crystal was fixed in the same position as formerly by the 

 method described towards the end of Section III. Then the observa- 

 tions recorded in Table III were taken consecutively. 



Table III. — Observations with a Polished Face. 

 Mean Temp. 14-85° C. 









K'. 



E. 



(*•' 



-r). 



(K'-K). 





91 -195° 



4-612° 



153 -714° 



61 -798° 



86 



•583° 



91 -916° 





91-089 



4-402 



153 -612 



61-936 



86 



•687 



91 -676 





90-866 



4-385 



153-545 



61 -954 



86 



•481 



91-591 





91-048 



4-325 



153-781 



62 -163 



86 



•723 



91-618 





91 • 162 



4-194 



153 -229 



62 -095 



86 



968 



91 -134 





90 -871 



4-400 



153 406 



61 -969 



86 



•471 



91 -437 





91 -008 



4-189 



153-376 



62 -095 



86 



•819 



91 -281 





90-884 



4-166 



153 -637 



62 -110 



86 



•718 



91-527 





91 -020 



4-191 



153-465 



62 -244 



86 



•829 



91 -221 





91-244 



4-061 



153 -620 



62 -287 



87 



•183 



91 -333 





91 '066 



4-118 



153 -736 



f2-295 



86 



•948 



91 -441 





90-907 



4-084 



153 -410 



62 -200 



86 



•823 



91 -210 



Means . . 



91 -030 



4-260- 



153-544 



62 -095 



86 



769 



91 -449 



