According to the elastic-solid theory of the ether, light consists of a periodic or wave-like disturbance in ajelly-like medium, the waves traveling in straight lines with a uniform veloc ity of about 186,000 miles per second, and the di rection of oscillation of the ether being at right angles to the direction in which the wave pro gresses, just as the direction of oscillation of the various points of a rope along which a wave is passing is at right angles to the rope. This view of the case accords very well with most of the observed phenomena, but there are some that do not appear to be reconcilable with it. We assume that the ether penetrates all bodies, and fills up the spaces between their molecules (or electrons) ; and as the phenomena of re fraction show that the velocity of light is less in a transparent solid (say in glass) than it is in a vacuum, it follows that the ether in the glass has either a greater density or a less rigidity than it has in free space. Either of these suppositions will fit this simple case equally well; but there are other phenomena that will not be satisfied so easily, and it has been found to be impossible to make any single set of consistent assumptions which shall recon cile the "elastic-solid" theory of the ether with all the known facts. For example, when we come to investigate certain problems in partial reflection from transparent media, and others relating to diffraction from small particles, we are obliged to conclude that it is the density of the ether varies, the rigidity remaining practically constant. On the other hand, the phenomena of double refraction require us to admit that the rigidity of the ether in a doubly refracting body is different in different direc tions; and hence we conclude that the rigidity of the ether is modified by the presence of mole cules of matter —a conclusion at variance with that previously reached by considering the phenomena of diffraction and partial reflec tion. Other difficulties have been encountered in the application of the elastic-solid theory of the ether to the phenomena of light, and al though reference to it is common, because it is definite enough to present a clear image to the mind, and so is helpful in many ways, the gen eral opinion among physicists of the present day is that it is no longer tenable as an accurate description of the real properties of the ether. It has been abandoned in favor of the *electro magnetic" theory of Maxwell, and in abandon ing it we also abandon his method of estimating the density and rigidity of the ether.
Faraday was convinced, many years ago, that there is some mechanism by which magnetic and electric forces are enabled to make themselves felt through a space apparently vacuous. *Such an action," he said, "may be a function of the ether; for it is not unlikely that, if there be an ether, it should have other uses than simply the conveyance of radiation.' Maxwell, after read ing Faraday's writings, became so impressed by the ideas which they advanced that he applied his own ingenious and powerful mind to the problems whose solution Faraday had dimly glimpsed, and succeeded in completely revolu tionizing our ideas with regard to light and the ether. His now famous "electro-magnetic the ory" is given in his masterly but exceedingly difficult 'Treatise on Electricity and Magnetism' and a popularized account of it may be found in Oliver J. Lodge's 'Modern Views of Electricity.' He agrees with previous writers that light is some sort of a periodic disturbance in some sort of an ether, and that the displacements that occur are indeed perpendicular to the direction in which the light-wave travels; but he teaches us that these displacements are not analogous to those that are produced in an elastic solid when that solid is deformed. He considers that they are of an electrical nature, and that we must learn about them not by observing the behavior of elastic bodies under stress, but by observing the phenomena exhibited by electrified bodies.
Maxwell has given us the fundamental equa tions that must be satisfied when an electrical disturbance is propagated through the ether, and by means of these equations the entire theory of light can be constructed on the new basis. The theory thus constructed agrees well with the facts of observation, and it is free from the objections that beset the old elastic solid theory. Moreover, it successfully with stood the searching experimental tests devised and executed by Hertz and his followers, whose labors have shown us that electrical radiations are propagated with the same speed as light, and that they can be reflected, refracted, dif fracted, polarized, and made to interfere; so that we are now quite ready to admit that light consists in a rapid succession of such radia tions. It is not at all essential to Maxwell's theory that we should know precisely what an *electrical displacement" really is, and hence it does not teach us so much about the nature of the ether as we might desire. It does teach that the elastic-solid analogy is probably not correct, and it strongly suggests that the ether is incompressible, and that there is some kind of an ethereal rotation going on in a magnetic field; but it has not yet been made to furnish a means of .e!timating the density of the ether, nor of obtaining any of its other constants.
We. do not even certainly know whether the ether is continuous, or whether it is molecular in. structure. Some writers find it difficult to think. of a .displacement of any lcind, in a space that is entirely filled with matter, especially if the matter is incompressible. Others hold that this objection is without weight.
Faraday's idea that magnetic and electric induction are propagated by the same medium as light proved to be exceedingly fruitful, and it is by no means unlikely that the ether pos sesses still other functions, which will throw further light upon its nature, when they are understood. The various lcinds of radiations that have been discovered in recent years (scathode rays,° °Becquerel rays,° °X-rays,° and the like) were at first believed by many au thorities to consist in ethereal motions different from those constituting light, and it was even thought that some of them might correspond to the waves of ethereal compression that had been so earnestly sought Some of these radia tions, however, are now believed to be nothing but ordinary light of exceedingly short wave length, and others are believed, at least tenta tively, to consist in the actual emission of storins of corpuscles, or °electrons,° from the bodies from which they proceed. (See EtEcraow ; RADIU M ; RADIATION ) . Gravitative action has also been attributed to ether stresses, and it is not impossible that this is its real nature. No mechanical explanation of gravitation, as an ether-phenomenon, has yet been offered, how ever, to which serious objections cannot be urged. In Maxwell's theory of gravitation it is assumed that bodies produce a stress in the ether about them, of such a nature that there is a pressure along the lines of gravitative force, combined with an equal tension in all directions at right angles to those lines. °Such a state of stress,' says Maxwell, "would no doubt account for the observed effects of gravitation. We have not, however, been able, hitherto, to imagine any physical cause for such a state of stress.° He calculates that to produce the actual effects of gravity, as observed at the sur face of the earth, the ether would have to be subject to a pressure of 37,000 tons per square inch in a vertical direction, and a tension of the same numerical magnitude in all horizontal directions.