446 On the Vibrations of Plane-Polarized Light. 



inversely proportional to the coiTcsponding velocities of trans- 

 mission. Then will the velocity of transmission of any trans- 

 verse \ibratoi'y movement be sensibly proportional to the reci- 

 procal of a diameter of that ellipsoid, clra^\^l parallel to the 

 direction of vibration. And if a plane be drawn through the 

 centre of the ellipsoid parallel to a series of plane-waves, the two 

 axes of the elliptic section so made will represent in magnitude 

 the reciprocals of the greatest and least normal velocities of 

 transmission of waves' parallel to that plane, and in direction, the 

 corresponding directions of vibration. 



This agrees exactly with the construction given by Presnel, 

 on which "his entire theory of double refraction is founded. 



The degree of symmetiy and uniformity of aiTangement of 

 the atoms w^hich is necessary in order that the mean intei-val 

 may have a definite value, and that three axes of distribution 

 may exist, is the same which is necessary to the existence of 

 rectangular axes of elasticity in a solid body. It must extend, 

 round each point, throughout a space which is large as compared 

 with the sphere of appreciable molecular action. 



The experiments of Sir David Brewster and of Fresnel on the 

 action of compressed glass on polarized light, show that rays 

 polarized in a plane normal to the direction of compression, that 

 is to say, vibrations parallel to that direction, are accelerated. 

 This indicates that the atmospheric load on each \abrating nucleus 

 in that direction is diminished, probably by the displacement of 

 a portion of the atmosphere out of that line. 



5. Though I have assumed in the course of this investigation 

 that the luminiferous medium is equally elastic in all directions, 

 I by no means intend to assert that it is necessarily so in all 

 substances ; but merely that, in most known crystalhne media, 

 an atmospheric load on the vibrating nuclei is the predominant 

 cause of variation in the velocity of transmission with the direc- 

 tion of transverse vibration. 



It is remarkable that Fresnel, in his theory of the intensity of 

 reflected and refracted light, speaks of the particles of the lumi- 

 niferous medium as being mox'e or less loaded in substances of 

 greater or less refractive power. He did not, however, apply 

 this idea to double refraction, although he adopted a theory, 

 which, as we have seen, results from it. 



The principles laid down in this paper are not compatible with 

 the prevalent idea of a luminiferous aether enveloping ponderable 

 particles. The fundamental idea from which they spring is the 

 converse : that the luminiferous medium is a system of atomic 

 nuclei or centres of force, W'hose office it is to give form to matter; 

 while the atmospheres by which they are sm-rounded give, of 

 themselves, merely extension. 



Glasgow, August 1850, 



