Page:Elementary Text-book of Physics (Anthony, 1897).djvu/410

396 The medium in which magnetic, electrical, and light phenomena take place is called the ether. It pervades all space within the bounds of the known universe, and is so far material that it can transmit energy from one material body to another; the manner of its connection with the atoms of matter is not well understood, and the question of the influence of matter upon it is one of the most obscure in modern physics. The ether was first represented by Fresnel as an elastic solid possessing a rigidity estimated by Thomson to be about the one-thousand millionth of that of steel, and a density estimated to be 9.36 X 10"" grams per cubic centimetre. Thomson has shown that the properties of the ether, at least those concerned in the transmission of light, may be explained by supposing it composed of minute material bodies rotating like gyroscopes about definite axes. The most interesting view of the ether is that recently proposed by Fitzgerald. He conceives of it as a continuous fluid filled with vortices. These vortices may be either infinitely long linear vortices threading past each other in all directions, or ring vortices interlinked with each other; Fitzgerald has shown that such an assemblage of vortices will transmit electromagnetic vibrations comparable in all respects to those of light. The connection of this theory with Thomson's theory of the vortex atom gives it additional interest.

324. Wave Surfaces.—In § 130 is explained the general mode of propagation of wave motion in accordance with Huygens' principle. When light emanating from a point proceeds with the same velocity in all directions, the wave fronts are evidently concentric spherical surfaces. There are, however, many cases, especially in crystalline bodies, of unequal velocities in different directions. In these cases the wave fronts are not spherical, but ellipsoidal, or surfaces of still greater complexity.

326. Straight Lines of Light.—When a small screen $$A$$ (Fig. 92) is placed between the eye and a luminous point, the luminous point is no longer visible. Light cannot reach the eye by the curved or broken line $$PAE$$, and is therefore said to move in straight lines. This seems not to accord with Huygens' principle,