Reflexion of Light from Iceland Spar. 



265 



polishing a natural face. Also, this inference is supported by two 

 data as regards the elliptic analyser, and by the results obtained with 

 the simple analyser. 



Sir John Conroy* has come to a different conclusion. " It did not 

 seem worth while to make any further experiments with artificial 

 surfaces, as it seemed certain the results would be untrustworthy." 



Conroy's experiments were made with the surface immersed in 

 water to secure a greater rotation. I am not sure that the greater 

 rotation would compensate for the loss of light which must ensue 

 when reflexion takes place at the surface of two media of nearly the 

 same density and refractive index. 



Another cause of error has been indicated by Conroy. There was 

 some uncertainty whether the face was polished parallel to itself. 

 Though it would not altogether account for the large changes 

 observed by Conroy, yet I found in the case of a crystal, with which I 

 performed a first set of experiments for two months, that a percep- 

 tible alteration of the inclination of the face was sufficient to reduce 

 the elliptically polarised light to plane polarised. Thus as regards 

 my own experiments such a change of inclination would have been 

 quite fatal, and I abandoned the crystal. 



There is another point which is worthy of attention. An extreme 

 ill-luck seems to have befallen those who have had their crystals 

 polished by others. Thus Seebeckf found that in the case of a glass 

 polished by an artist there was a difference of 28" 5', but that he 

 himself could polish it so that the difference was only 2'9'. I think 

 most likely this may be the probable cause of the large alterations 

 observed by Sir J. Conroy. 



XV. Experiments with a Cleavage Face 'parallel to the Rejpolished Face. 



To make the whole series of experiments as conclusive as possible, I 

 now attempted to make a series of experiments with a natural face as 

 at the beginning of the paper. Unfortunately the crystal did not 

 cleave very readily. In splitting the base of the crystal came into two 

 or three pieces, which were fixed on to it again as accurately as 

 possible in their former position. As the sides of the crystal also came 

 off, the inclination of the face to the repolished face could not be deter- 

 mined. There was some doubt as to whether the face of the crystal 

 was in the same absolute position. The surface was not very good, 

 being somewhat broken, and as the reflexion took place from a part 

 of the crystal nearer an edge, it had to be placed so much on one side 

 of the brass table that the template could not be used. Then the 

 observations recorded in Table XV were taken. 



VOL. XLII. 



* ' Boy. Soc. Proc.,' Feb., 1886. 

 t ' Poggendorff, Aimaleri,' vol. 20. 



U 



