
Cara!) 
XXV.—Experimenis on the Ordinary Refraction of Iceland Spar. By Wiu1am 
Swan, Esq. Communicated by Professor KELLAND. 
(Read 19th April 1847.) 
According to the theory devised by Huycens, 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 light 
to the optical axis of the crystal. 
Dr Wottaston, in 1802, verified the spheroidal form of the wave of light, 
which Huyerns 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 reflec- 
tion.”* In 1810, Matus, in his Théorie de la Double Réfraction, also demonstrated 
experimentally the accuracy of the Huygenian law for the extraordinary pencil. 
I have not had an opportunity of consulting the memoir of Matus, so as to 
know the precise nature of his experiments, with reference to the refraction of 
the ordinary ray; but the object of Dr Wotiasron’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 MaccutLacu of Dublin, in order to account for cer- 
tain phenomena observed by Sir Davip BREewsTER, 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 Davip BREWSTER 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 their 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 Davin BrewstER.—Notices 
and Abstracts of Communications of the British Association, 1848, p. 7. 
VOL. XVI. PART III. 5¢ 
