Another even more fatal objection to the cor puscular theory was, that this theory cannot explain refraction unless it is admitted that light travels faster in a dense medium than it does in a rarer one, while the undulatory theory requires that the light shall move faster in the rarer medium. Fizeau and Foucault proved by direct experiment that light travels faster in air than it does in water; and hence it was considered that they had proved that the cor puscular theory is not correct.
About the middle of the 19th century Michael Faraday published his experimental researches on electricity and magnetism; and although these did not deal particularly with light, it happened that they had a most import ant ultimate influence upon our views as to its nature. Maxwell, in studying Faraday's work and endeavoring to reduce his experi mental results to a consistent mathematical theory, conceived the idea that light may be merely a periodic or vibratory electrical dis turbance in the ether. Upon working out this idea in detail, Maxwell found that the new theory is free from many of the objections that had developed in connection with the older elastic-solid theory; and the "electro-magnetic theoryp of Maxwell is now held by nearly all physicists, in preference to the older undula tory theory. The difference between the two
is somewhat as follows: Maxwell agrees that light is some sort of a periodical disturbance in some sort of an ether, and he also agrees that the displacements that occur as the wave pro gresses are perpendicular to the direction in which the wave travels; but he teaches 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. In one sense, therefore, Maxwell's theory may still be regarded as a form of the undulatory theory; hut the waves that are contemplated by it are electrical in nature, and are not strictly analogous to the waves in an incom pressible elastic solid. See ETHER; LIGHT; RADIATION; RADIOACTIVITY; X-RAY.