According to the theory devised by Huygens, to explain the phenomenon of double refraction in Iceland spar, a pencil of light transmitted through that substance is divided into two pencils; the index of refraction for the one being constant, while for the other it varies with the inclination of the transmitted light 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 reflection.” In 1810, Malus, 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 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 refraction for the ordinary ray, is therefore tacitly assumed by him.