VIBRATIONS OF ORDINARY LIGHT.

1049

in plane-polarized light is normal to the so-called plane of polarization, and therefore that, in polarization by reflection, the vibrations of the reflected light are parallel to the reflecting surface.

This is Fresnel's doctrine. MacCullagh, however, reversed this hypothesis, and maintained that the direction of vibration is in the plane of polarization. Both theories have been ably expounded; but Stokes contrived a crucial experiment in diffraction, which confirmed Fresnel's view;1 and in his classical paper on "Change of Refrangibility," he has deduced the same conclusion from a consideration of the phenomena of the polarization of light by reflection from excessively fine particles of solid matter in suspension in a liquid.2

842. Vibrations of Ordinary Light.-Ordinary light agrees with circularly-polarized light in always yielding two beams of equal intensity when subjected to double refraction; but it differs from circularly-polarized light in not becoming plane-polarized by transmission through a Fresnel's rhomb or a quarter-wave plate. What, then, can be the form of vibration for common light? It is probably very irregular, consisting of ellipses of various sizes, positions, and forms (including circles and straight lines), rapidly succeeding one another. By this irregularity we can account for the fact that beams of light from different sources (even from different points of the same flame, or from different parts of the sun's disc), cannot, by any treatment whatever, be made to exhibit the phenomena of mutual interference ; and for the additional fact that the two rectangular components into which a beam of common light is resolved by double refraction, cannot be made to interfere, even if their planes of polarization are brought into coincidence by one of the methods of rotation above described.

Certain phenomena of interference show that a few hundred consecutive vibrations of common light may be regarded as similar; but as the number of vibrations in a second is about 500 millions of millions, there is ample room for excessive diversity during the time that one impression remains upon the retina.

843. Polarization of Radiant Heat.-The fundamental identity of radiant heat and light is confirmed by thermal experiments on polarization. Such experiments were first successfully performed by Forbes in 1834, shortly after Melloni's invention of the thermomultiplier. He first proved the polarization of heat by tourmaline;

1 Cambridge Transactions. 1850.

2 Philosophical Transactions, 1852; pp. 530, 531.

next by transmission through a bundle of very thin mica plates, inclined to the transmitted rays; and afterwards by reflection from the multiplied surfaces of a pile of thin mica plates placed at the polarizing angle. He next succeeded in showing that polarized heat, even when quite obscure, is subject to the same modifications which doubly refracting crystallized bodies impress upon light, by suffering a beam of heat, after being polarized by transmission, to pass through an interposed plate of mica, serving the purpose of the plate of selenite in the experiment of § 833, the heat traversing a second mica bundle before it was received on the thermo-pile. As the interposed plate was turned round in its own plane, the amount of heat shown by the galvanometer was found to fluctuate just as the amount of light received by the eye under similar circumstances would have done. He also succeeded in producing circular polarization of heat by a Fresnel's rhomb of rock-salt. These results have since been fully confirmed by the experiments of other observers.