Oscillatory Theory of Light. 405 



sity of plane-polarized light reflected from singly refracting 

 substances. 



But it was afterwards demonstrated, that the elastic forces 

 which propagate a transverse movement in any medium must 

 necessarily be functions, not merely of the direction of the move- 

 ment, but jointly of this direction and the direction of propaga- 

 tion ; that is to say, of the position of the plane containing these 

 two directions. Consequently, if the various velocities of vari- 

 ously polarized light in a doubly refracting medium arise from 

 variations of elasticity in different directions, the direction of 

 vibration is in the plane of polarization, contrary to the hypo- 

 thesis of Fresnel. FresnePs wave-surface, and his approximate 

 formulae for the intensity of reflected light, are deducible from 

 this supposition as readily as from his original hypothesis ; and 

 Mr. MacCullagh obtained from it formula? for the intensity of 

 light reflected from doubly refracting substances, agreeing closely 

 with the experiments of Sir David Brewster. 



On the other hand, the formulae of M. Cauchy, and those of 

 Mr. Green as modified by Mr. Haughton*, expressing the 

 effects of reflexion on the intensity and phase of polarized light, 

 all of which are founded on the supposition that the direction of 

 vibration is perpendicular to the plane of polarization, have been 

 shown to be capable, by the introduction of proper constants, of 

 giving results agreeing closely with those of the important expe- 

 riments of M. Jamin (Annates de Chimie et de Physique, 3rd 

 series, vol. xxix. 1850) ; and it is difficult, if not impossible, to 

 see how such formulae could have been deduced from the oppo- 

 site supposition. 



But the true crucial experiment on this subject has been fur- 

 nished by the researches of Professor Stokes on the Diffraction 

 of Polarized Light (Camb. Trans, vol. ix.). Whatsoever maybe 

 the nature of the motion that constitutes light, if it can be 

 expressed by a function of the direction and length of a line 

 perpendicular to the direction of propagation (which may be 

 called a transversal), it is certain that this motion will be more 

 abundantly communicated round the edge of an obstacle when 

 its transversal is parallel than when it is perpendicular to that 

 edge ; so that the effect of diffraction is, to bring every oblique 

 transversal into a position more nearly parallel to the diffracting 

 edge. But it has been shown by the experiments of Professor 

 Stokes, that the effect of diffraction upon every ray of light 

 polarized in a plane oblique to the diffracting edge, is to bring 

 the plane of polarization into a position more nearly perpendicular 

 to the diffracting edge. Therefore the transversal of a ray of 

 plane-polarized light (which, if light consists in linear vibratory 

 * Philosophical Magazine, August 1853. 



