Page:Elementary Text-book of Physics (Anthony, 1897).djvu/409

§ 323] of a fluid, in which only longitudinal waves, like those of sound, can exist, and they were therefore inadequate to explain polarization, which shows that light in some way differs in different directions in the wave front, or, as Newton expressed it, has sides; and further, since observation and theory, so far as it was then developed, showed that a wave which has passed through an opening will spread in all directions from that opening and will not be propagated in a line, as light is, Newton rejected the wave theory, and developed the emission theory by assigning such properties to the light corpuscles as were needed to explain the facts then known.

The discovery by Young of interference, which can be most easily explained by the wave theory, and the demonstration by Fresnel that rectilinear propagation can be explained by taking into account the shortness of the wave length, and the further bold assumption by Fresnel that the properties of the medium are those of a solid and that the vibrations in the light wave are transverse to the line of progress, removed the objections which had been felt by Newton and set the wave theory on a secure foundation. The one objection which was still felt arose from the necessity of assuming the existence of an all-pervading medium, which cannot be made evident to any of our senses and which possesses properties unlike those of any known body. This objection has gradually disappeared in view of the almost complete success attained by the wave theory in explaining the phenomena of light. The demonstration by Maxwell that all magnetic and electrical phenomena can be explained by actions in a similar medium, and that the properties of electromagnetic waves in such a medinm are precisely the same as those of light waves, has done much to strengthen the evidence for the existence of this medium. Its properties are probably not those of an elastic solid, but they are such as to enable us to represent light waves as if they were waves in an elastic solid; we will accordingly, in what follows, use this mode of representation, with the understanding that the rigidity and density which are ascribed to the medium are representative of other properties of the medium, which, in respect to light waves, are equivalent properties.