408 Mr. W. J. M. Rankine's General View of an 



logous to that demonstrated by Professor Stokes for a transverse 

 vibratory movement, substituting only the axis of oscillation for 

 the direction of vibration ; that is to say, 



The direction of the axes of oscillation in the diffracted wave is 

 the projection of that of the axes of oscillation in the incident wave 

 on a plane tangent to the front of the diffracted wave. 



Consequently, oscillations in the incident wave, round axes 

 oblique to the diffracting edge, give rise to oscillations in the 

 diffracted wave round axes more nearly parallel to the diffracting 

 edge. 



But the experiments of Professor Stokes have proved, that 

 light polarized in a plane oblique to the diffracting edge, becomes, 

 after diffraction, polarized in a plane more nearly perpendicular to 

 the diffracting edge. 



Therefore the axes of oscillation in plane-polarized light are 

 perpendicular to the plane of polarization. 



Therefore the velocity of transmission of oscillations round 

 transverse axes through the luminiferous medium in a crystalline 

 body is a function simply of the direction of the axes of oscillation. 



Now if the variations of the velocity of transmission arose from 

 variations of the coefficient of transverse polarity (denoted by C), 

 they would depend on the direction of propagation as well as 

 upon that of the axes of oscillation, so that the plane of polar- 

 ization would be that which contains these two directions. Since 

 the velocity of transmission depends on the direction of the axes 

 of oscillation only, it follows that its variations in a given cry- 

 stalline medium arise wholly from variations of the moment of 

 inertia of the luminiferous atoms, together with their loads of ex- 

 traneous matter. 



Consequently the coefficient of polarity C for transverse axes 

 of oscillation is the same for all directions in a given substance. 



To account for the known laws of the intensity and phase of 

 reflected and refracted light consistently with the hypothesis of 

 oscillations, it is necessary to suppose also that this coefficient is 

 the same for all substances ; so that the variations of the velo- 

 cities of light and indices of refraction for different media depend 

 solely on those of the moments of inertia of the loaded lumini- 

 ferous atoms. 



There is reason to anticipate, that, upon further investigation, 

 it will appear that this condition is necessary to the stability of 

 the luminiferous atoms. 



4. Of the Wave- surface in Crystalline Bodies. 

 Let the axes of coordinates be those of molecular symmetry 

 in a crystalline medium. 



Let M„ Mj, M 3 be coefficients proportional to the moments 



