Oscillatory Theory of Light. 407 



lation ; — and inversely, to the square root of the sum of the 

 moments of inertia round the axes of oscillation of the atoms 

 contained in a given space, loaded with such portions of mole- 

 cular atmospheres surrounding them as they may carry along 

 with them in their oscillations. 



Then denoting by 



h, the velocity, in a given direction of plane-waves, of oscilla- 

 tion round transverse axes parallel to a given line ; 



C, a coefficient of polarity or rotative force for the given direc- 

 tions of propagation and of axes ; 



M, a coefficient of moment of inertia for the given direction 

 of axes ; 

 the above principle may be represented by this equation, 



The coefficient of polarity in question is proper only to an 

 axis of oscillation transverse to the direction of propagation. To 

 account for the stability of direction of the axes of the atoms, 

 and also for the non-appearance, in ordinary cases, of pheno- 

 mena capable of being ascribed to oscillations round axes parallel 

 to the direction of propagation, it is necessary to suppose the 

 corresponding coefficient for the latter species of oscillations to 

 be much greater than the coefficient for transverse axes of oscil- 

 lation. 



It is evident, that how powerful soever the polarity may be, 

 which is here ascribed to the atoms of the luminiferous medium, 

 it is a kind of force which must be absolutely destitute of direct 

 influence on resistance to change of volume or change of figure 

 in the parts of that medium, or of any body of which that medium 

 may form part ; and that, consequently, the difficulty which in 

 the hypothesis of vibrations arises from the necessity of ascribing 

 to the luminiferous medium properties like those of an elastic 

 solid, has no existence in the hypothesis of oscillations now 

 proposed. 



The luminiferous atoms may now be supposed to be diffused 

 throughout all space, and as molecular nuclei, throughout all 

 bodies ; the distribution and motion of their centres being regu- 

 lated by forces wholly independent of that species of polarity 

 which is the means of transmitting a state of oscillation round 

 those centres. 



3. Of the Diffraction of Plane-polarized Light, and the relation of 

 Axes of Oscillation to Planes of Polarization. 



In the diffraction of an oscillatory movement round transverse 

 axes past the edge of an obstacle, a law holds good exactly ana- 



