Reflexion of Light from Iceland Spar, 



255 



time, for otherwise tlie experiments made would not have very great 

 value unless the time elapsed since polishing were specified. I found 

 that preceding experimenters had made investigations on this point 

 in regard to a few bodies, and had come to the conclusion that the 

 surface did not alter with the time. Thus Seebeck found such a re- 

 sult for some glass experimented upon,* and Sir John Conroy has 

 proved that in the case of metallic surfaces the surface state is a 

 fairly permanent one, not being destroyed by contact with a liquid or 

 a considerable amount of rubbing with a chamois leather, f It there- 

 fore naturally occurred to me to examine this question for Iceland 

 spar. 



For this purpose a simple analyser consisting of a Nicol and a 

 graduated circle was set up on the side of the table opposite to the 

 elliptic analyser, and placed in such a position that the light could be 

 reduced to a minimum. The observations taken are recorded in 

 Table VIII. 



Table VIII. 



Date. 



Temp. 



No. of 

 readings. 



Mean reading 

 of 1st vernier. 



Mean reading 

 of 2nd yernier. 



Mean of 

 verniers. 



Dec. 8-11 . 



9 ? 5 C. 



40 



2°1 10'-9 



201 20 / *5 



Ill 15-7 



Jan. 20 . . . 



9-7 



60 



21 11-76 



201 21 -6 



111 16*68 





9-8 



60 



21 11 -93 



201 20-8 



111 16-36 



Thus there is in the period of six weeks no time variation of a 

 polished surface, for the differences between the means are quite 

 within the limits of experimental error. Also this result is confirmed 

 by the general character of the observations made with the elliptic 

 analyser, which often extended over some weeks, during which no 

 change was noticed. 



XI. Variation of Surface State of a Polished Crystal with Change of 



Temperature. 



An attempt was made to secure greater accuracy by altering the 

 arrangement of the parts of the elliptic analyser. For this purpose 

 the screw fixing the vertical stem was loosened, and the circular rim 

 rotated through two right angles about the stem as axis. The Nicol 

 was unscrewed from its collar, and the cell containing the quarter 

 wave plate from the disk. The quarter wave plate was now con- 

 nected with the collar and the Mcol with the disk. In this mode of 



* 1 Poggendorff, Annalen,' vol. 20. 

 f ' Eoy. Soe. Proc.,' vol. 31, 1881. 



