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XXV. — Experiments on the Ordinary Refraction of Iceland Spar. By William 

 Swan, Esq. Communicated by Professor Kei-land. 



(Read 19th April 1847.) 



According to the theory devised by Huygens, to explain the phenomenon of 

 double refraction in Iceland spar, a pencil of light transmitted through that sub- 

 stance is divided into two pencils ; the index of refraction for the one being con- 

 stant, while for the other it varies with the inclination of the transmitted ho-ht 

 to the optical axis of the crystal. 



Dr WoLLASTON, in 1802, verified the spheroidal form of the wave of light, 

 which Huygens had assumed to account for the refraction of the extraordinary 

 pencil, by a careful experimental investigation, conducted by means of his elegant 

 instrument for " examining refractive and dispersive powers by prismatic reliec- 

 tion."* In 1810, Malus, in his Theorie de la Double Refraction, also demonstrated 

 experimentally the accuracy of the Huygenian law for the extraordinary pencil. 

 1 have not had an opportunity of consulting the memoir of Malus, so as to 

 know the precise nature of his experiments, with reference to the refraction of 

 the ordinary ray; but the object of Dr Wollaston's researches was simply to 

 prove the law of extraordinary refraction, and the constancy of the index of re- 

 fraction for the ordinary ray, is therefore tacitly assumed by him. 



More recently. Professor Maccullagh of Dublin, in order to account for cer- 

 tain phenomena observed by Sir David Beewstee, in the reflexion of light from 

 Iceland spar, was led to propose a law of double refraction, according to which 

 the ordinary ray in that substance has a variable index of refraction ; and at his 

 request. Sir David Beewstee made an experiment to ascertain whether the 

 ordinary refraction of Iceland spar is different at different inclinations to the 

 axis. Two prisms were cut out of the same piece of spar, so that in one the 

 transmitted ray was at right angles to the axis, and in the other, it was coinci- 

 dent with it ; and both being cemented to a plate of glass, had then- surfaces 

 ground and polished together, so as to ensure the equality of their refracting 

 angles. It was then found that the images of a narrow slit, illuminated by 

 homogeneous yellow light, seen through the prisms, were perfectly coincident, 

 which proved that the index of refraction for the ordinary ray was the same in 

 both prisms, " within the limits of the errors of observation."! 



* Philosophical Transactions, 1802, pp. 365 and 387. 



\ See Experiment on the ordinary refraction of Iceland spar, by Sir David Beewster. — Notices 

 and Abstracts of Communications of the British Association, 1843, p. 7. 



VOL. XVI. PART III. 5 C 



