ON THE NATURE OF LIGHT AND COLOURS. 371 



[gether obliterates the light of the other colour. Nothing can be more 

 satisfactory than the explanation which the theory affords of such pheno- 

 mena, and, whilst we do not assert that it has as yet brought every 

 observed fact within its pale, yet it does not appear that the arguments 

 which were raised against it have any power to shake it. Regarding it 

 as true, we shall adopt its language in giving a very brief sketch of the 

 phenomena of polarized light. 



It has been stated that when a pencil of light falls on a surface of Ice- 

 land spar, it is divided into two. Huygens, by the hypothesis that one 

 series of waves diverges into a spheroid, whilst the other diverges into a 

 sphere, gave a most satisfactory explanation of the course of the two rays ; 

 and his conclusions were confirmed by the accurate measurements of Wol- 

 laston.* Dr. Young t perceived that this difference of divergence must 

 arise from a difference of elasticity within the crystal. Combining this 

 with the idea of Newton, that a ray of light possesses sides, the hypothesis 

 of a transverse vibration is a natural result. Dr. Young advanced this 

 hypothesis about 1817, and from that period the progress of the theory 

 has been rapid and satisfactory. The hypothesis consists in supposing 

 that the particles of light do not, like those whose motions constitute sound, 

 oscillate in the direction of the wave, but transversely to it, so as more to 

 resemble those of the particles of water which move up and down whilst 

 the wave advances horizontally. The explanation of double refraction is 

 now quite simple. A ray of light falls on the surface of a crystal, the 

 elasticity of which is different in different directions. The motions, con- 

 sequently, are not all transmitted with the same velocity, and as the index 

 of refraction depends on the velocity, one set of vibrations will, on emer- 

 gence, be totally separated from another. Moreover, the light on emerging 

 is quite different from common light. In each ray it consists only of vibra- 

 tions in one direction. Suppose, therefore, one of these rays to fall on a 

 second crystal placed in a similar position with the first, it will not now be 

 divided into two, but will emerge just as it entered. Light which consists of 

 vibrations in one direction only is termed polarized light. It was discovered 

 by Malus that light reflected from the same face of unsilvered glass is more 

 or less polarized ; and Brewster ascertained that it is perfectly so, when the 

 tangent of the angle of incidence is equal to the refractive index, and also 

 that the transmitted ray is partially polarized. Moreover, Seebeck and Biot 

 discovered a property of the tourmaline, that when it is cut into slices, 

 whose surfaces are parallel to the axis of the crystal, it absorbs one of the 

 two rays, and consequently transmits a polarized ray only. Thus we are 

 presented with various ways of effecting the polarization of light. The 

 simplest to understand is that by the tourmaline, and to it we shall conse- 

 quently refer. On looking through a plate of tourmaline, the effect to the 

 eye is similar to that produced by a bit of coloured glass. If a second 

 plate of tourmaline be placed on the first, so that their axes are parallel to 

 each other, the same is true. But if the axis of the one be perpendicular 

 to that of the other, the one horizontal, the other vertical, the compound 

 plate becomes opaque. The first suffering only horizontal vibrations to] 

 * Ph. Tr. 1802, p. 381. f Quarterly Review, 1809, ii.344. 



2B2 



