Reflexion of Light from Iceland Spar. 



253 



The value of p which has been determined is for red light, whereas 

 the experiments made on the crystal face were performed with white 

 light. It was not possible to determine p accurately for blue rays, as 

 a blue glass absorbed too much light. We cannot, therefore, employ 

 the value of p as determined by the steel plate to accurately correct 

 the observations made with the crystal. Nevertheless, let us see what 

 the effect of the substitution will be. 



We have cos 2-sr 



and cos p = 



sin (r' — r) 

 sin (R'-R/ 



tan (r r — r) 

 tan (R' — R)' 



Since p is supposed known, eliminate the smaller quantity R'—R, and 

 we find 



sin 2-ar = cos (r'—r) sin p (A.) 



This formula shows that if there be any change in the ratio of the 

 axes there must be a change in the value of r'—r. Looking at the 

 Tables I, III, we see there is such a change. 



Using the last written formula and the mean values of r — r from 

 Tables I, III, we find that for a natural face 



tan *r = 0-03552, 



and for the polished face 



tan *r = 0-02789. 



Thus, even when the value of p for red light as determined by the 

 steel plate is used, we find that the numerical results indicate that the 

 character of the change produced by polishing remains the same. 



In the case of the natural face, taking 12 observations of r' — r from 

 Table I, the greatest deviation from the mean is less than 9', leading 

 by formula A to a deviation from the mean of less than 5 per cent. 

 In the case of the polished face, the greatest deviation will be, accord- 

 ing to Table III, about 10 per cent. 



The value of p has been determined for only one colour, and conse- 

 quently we cannot calculate its value accurately for light of mean wave- 

 length, so as to be able to compare the value of p obtained from the 

 steel plate with the value of p obtained from observations with the 

 crystal. Let us, however, examine in a general way what the effect 

 of change of wave-length on the value of p is. 



Let t be the thickness of the selenite plate, ju x , jx' x the ordinary and 

 extraordinary indices for a line x of the spectrum. 



