Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/593

Rh E T H E 11 571 deduced from the observation of the eclipses of Jupiter s satellites when Jupiter is seen from the earth at nearly opposite points of the ecliptic. Arago proposed to compare the deviation produced in the light of a star after passing through an achromatic prism when the direction of the ray within the prism formed different angles with the direction of motion of the earth in its orbit. If the aether were moving swiftly through the prism, the deviation might be expected to be different when the direction of the light was the same as that of the tether, and when these directions were opposite. The present writer 1 arranged the experiment in a more practicable manner by using an ordinary spectroscope, in which a plane mirror was substituted for the slit of the col- limator. The cross wires of the observing telescope were illu minated. The light from any point of the wire passed through the object-glass and then through the prisms as a parallel pencil till it fell on the object-glass of the collimator, and came to a focus at the mirror, where it was reflected, and after passing again through the object-glass it formed a pencil passing through each of the prisms parallel to its original direction, so that the object-glass of the observing telescope brought it to a focus coinciding with the point of the cross wires from which it originally proceeded. Since the image coincided with the object, it could not be observed directly, but by diverting the pencil by partial re flection at a plane surface of glass, it was found that the image of the finest spider line could be distinctly seen, though the light which formed the image had passed twice through three prisms of 60. The apparatus was first turned so that the direction of the light in first passing through the second prism was that of the earth s motion in its orbit. The apparatus was afterwards placed so that the direction of the light was opposite to that of the earth s motion. If the deviation of the ray by the prisms was increased or diminished for this reason in the first journey, it would be diminished or increased in the return journey, and the image would appear on one side of the object. When the apparatus was turned round it would appear on the other side. The experiment was tried at different times of the year, but only negative results were obtained. We cannot, however, conclude absolutely from this experiment that the aether near the surface of the carih is carried along with the earth in its orbit, for it has been shown by Professor Stokes 2 that according to Fresnel s hypothesis the relative velocity of the father within the prisrn would be to that of the nether outside inversely as the square of the index of refraction, and that in this case the deviation would not be sensibly altered on account of the motion of the prism through the osther. Fizeau, 3 however, by observing the change of the plane of polarization of light transmitted obliquely through a series of glass plates, obtained what he supposed to be evidence of a difference in the result when the direction of the ray in space was different, and Angstrom obtained analogous results by diffraction. The writer is not aware that either of these very difficult experiments has been verified by repetition. Tn another experiment of M. Fizean, which seems entitled to greater confidence, he has observed that the propagation of light in a stream of water takes place with greater velocity in the direction in which the water moves than in the opposite direction, but that the change of velocity is less than that which would be due to the actual velocity of the water, and that the phenomenon does not occur when air is substituted for water, This experiment seems rather to verify Fresnel s theory of the a-iher ; but the 1 Phil. Trans., clviii. (1SG8), p. 5o2. 4 Phil. Mar,., 1846, p. 53. 3 Ann. dc Chimie et de Physique, Felt. 1800. whole question of the state of the luminiferous medium near the earth, and of its connexion with gross matter, is very far as yet from being settled by experiment. Function of the aether in electromagnetic phenomena. Faraday conjectured that the same medium which is concerned in the propagation of light might also be the agent in electromagnetic phenomena. &quot; For my own part,&quot; he says, &quot; considering the relation of a vacuum to the magnetic force, and the general character of magnetic phenomena external to the magnet, I am much, more inclined to the notion that in the transmission of the force there is such an action, external to the magnet, than that the effects are merely attraction and repulsion at a dis tance. Such an action may be a function of the aether; for it is not unlikely that, if there be an tether, it should have other uses than simply the conveyance of radia tion. 4 This conjecture has only been strengthened by subsequent investigations. Electrical energy is of two kinds, electrostatic and electrokinetic. We have reason to believe that the former depends on a property of the medium in virtue of which an electric displacement elicits an electromotive force in the opposite direction, the electromotive force for unit displacement being inversely as the specific inductive capa city of the medium. The electrokinetic energy, on the other hand, is simply the energy of the motion set up in the medium by electric currents and magnets, this motion not being con fined to the wires which carry the currents, or to the mag net, but existing in every place where magnetic force can be found. Electromagnetic Theory of Liykt. The properties of the electromagnetic medium are therefore as far as we have gone similar to those of the Juminiferous medium, but the best way to compare them is to determine the velocity with which an electromagnetic disturbance would be propagated through the medium. If this should be equal to the velocity of light, we would have strong reason to believe that the two media, occupying as they do the same space, are really identical. The data for making the calculation are furnished by the experiments made in order to compare the electromagnetic with the electrostatic system of units. The velocity of propagation of an electromagnetic disturbance in air, as calculated from different sets of data, does not differ more from the velocity of light in air, as determined by different observers, than the several calculated values of these quantities differ among each other. If the velocity of propagation of an electromagnetic disturbance is equal to that of light in other transparent media, then in non-magnetic media the specific inductive capacity should be equal to the square of the index of refraction. Boltzmann 5 ha? found that this is very accurately true for the gases which he has examined. Liquids and solids exhibit a greater divergence from this relation, but we can hardly expect even an approximate verification when we have to compare the results of our sluggish electrical experiments with the alternations of light, which take place billions of times in a second. The undulatory theory, in the form which treats the phenomena of light as the motion of an elastic solid, is still encumbered with several difficulties. 6 The first and most important of these is that the theory indicates the possibility of undulations consisting of vibra tions normal to the surface of the wave. The only way of 4 Experimental Researches, 3075. 5 Keener Silzb., 23 April 1874. 6 See Prof. Stokes, &quot;Report on Double Refraction,&quot; British Afs. Report, 1802, p. 253.