202 



Mr. R. T. Glazebrook. 



[June 19, 



XVI. " Double Refraction and Dispersion in Iceland Spar. An 

 Experimental Investigation with a Comparison with Huy- 

 ghens' Construction for the Extraordinary Wave." By R. 

 T. Glazebrook, M.A., Fellow of Trinity College, Cam- 

 bridge. Communicated by J. Clerk Maxwell, M.A., F.R.S. 

 Received June 12, 1879. 



(Abstract.) 



In the " Proceedings of the Royal Society," vol. xxvii, p. 498, the 

 results of some experiments on double refraction in aragonite are given. 

 From them I was led to the conclusion that there were slight differences 

 between Fresnel's theory and experiment, and that possibly there was 

 some connexion between these differences and the wave-length of the 

 light used. 



The present paper, of which this is an abstract, is an account of 

 the results of similar experiments on Iceland spar. The method of the 

 experiments is due to Professor Stokes ("Brit. Assoc. Reports," 

 1862). 



A prism is cut from the crystal to be examined, and the position of 

 its faces with reference to the crystallographic axes carefully deter- 

 mined. Then we may show with the usual notation that 



tan tan* cot ^ tan . . . (I). 



2 2 2 2 w 



From this by observing the angles of incidence and deviation of a 

 pencil of parallel rays incident on a prism of known refracting angle 



we can find — — and hence get 0' or ty' • then from the formula 

 2 



V sin 



v sin 0' 



V being the velocity of normal propagation in air, v in the crystal, 

 we can find v in certain definite directions. But Huyghens' con- 

 struction enables us to determine from theory the value of v, and we 

 have hence a method of comparison. 



Professor Stokes has made a series of experiments on Iceland spar 

 by this method, and stated his results in " Proceedings of the Royal 

 Society," vol. xx, 443. He found that to the limit of the error of his 

 observations (about 0"0001 in the index) Huyghens' construction was 

 fully confirmed. 



The details of his experiments have not been published, and it ap- 



(ii). 



