The Aether as an Elastic Solid. 169 



diffraction, is so transmitted, the plane of vibration will be more 

 nearly at right angles to the plane of diffraction in the diffracted 

 than in the incident light. Stokes himself performed experi- 

 ments to test the matter, using a grating in order to obtain 

 strong light diffracted at a large angle, and found that when 

 the plane of polarization of the incident light was oblique to the 

 plane of diffraction, the plane of polarization of the diffracted 

 light was more nearly parallel to the plane of diffraction. This 

 result, which was afterwards confirmed by L. Lorenz,* appeared 

 to confirm decisively the hypothesis of Fresnel, that the vibra- 

 tions of the aethereal particles are executed at right angles to 

 the plane of polarization. 



Three years afterwards Stokes indicatedf a second line of 

 proof leading to the same conclusion. It had long been known 

 that the blue light of the sky, which is due to the scattering of 

 the sun's direct rays by small particles or molecules in the 

 -atmosphere, is partly polarized. The polarization is most 

 marked when the light comes from a part of the sky distant 90 

 from the sun, in which case it must have been scattered in a 

 direction perpendicular to that of the direct sunlight incident 

 on the small particles ; and the polarization is in the plane 

 through the sun. 



If, then, the axis of y be taken parallel to the light incident 

 on a small particle at the origin, and the scattered light be 

 observed along the axis of x, this scattered light is found to be 

 polarized in the plane xy. Considering the matter from the 

 dynamical point of view, we may suppose the material particle 

 to possess so much inertia (compared to the aether) that it is 

 practically at rest. Its motion relative to the aether, which is 

 the cause of the disturbance it creates in the aether, will there- 

 fore be in the same line as the incident aethereal vibration, 

 but in the opposite direction. The disturbance must be 

 transversal, and must therefore be zero in a polar direction and 



* Ann. d. Phj-s. exi (1860), p. 315. Phil. Mag. xxi (1861), p. 321. 

 t Phil. Trans., 1852, p. 463. Stokes's Math, and Phys. Papers, iii, p. 267. 

 f. the foot-note added on p. 361 oi the Math, and Phys. Paper*. 



