AND THE ETHER SUPPOSED TO PERVADE THE WHOLE UNIVERSE. ABOUT two hundred years ago Descartes, Hook, and Huygens, three of the most celebrated mathematicians of their day, entertained the idea that light was propagated by the vibrations and undulations of a subtile elastic fluid called ether, which not only filled infinite space, but was contained in all solid, fluid, and gaseous bodies. The immortal Newton, who was opposed to this theory, or at least created one of his own, usually called the Corpuscular Theory of Light, appears to have entertained the opinion (according to Enfield) that "All fixed bodies, when heated beyond a certain degree, emit light and shine; and this emission is performed by the vibrating motion of their parts." "The heat of a warm room is conveyed through a vacuum by the vibration of a much subtiler medium than air, which, after the air is drawn out, remains in the vacuum. "It is by the vibrations of this medium that light is refracted and reflected, and heat communicated. This medium is exceedingly more elastic and active, as well as subtile, than the air; it readily pervades all bodies, and is by its elastic force expanded through the heavens. Its density is greater in free and open space than in compact bodies, and increases as it recedes from them. This medium, growing denser and denser perpetually as it passes from the celestial bodies, may, by its elastic force, cause the gravity of those great bodies towards one another, and of their parts towards the bodies. Vision, hearing, and animal motion may be performed by the vibrations of this subtile elastic fluid or ether." These opinions would seem to show that Newton believed all emanations of particles of light were attended by the undulations of an ethereal medium accompanying it in its passage. The theory, however, generally ascribed to him is, that rays of light are small corpuscles emitted with exceeding celerity, travelling at about the rate of one hundred and eighty-two thousand miles per second; and these rays of light, falling upon the eye, excite vibrations in the tunica retina, which, being propagated along the solid fibres of the optic nerve to the brain, cause the sense of sight. Could Newton, who insisted so much on the importance of experimenting before enunciating a theory, have been acquainted with the highly interesting experiments connected with the inflection or diffraction of light, he would not have opposed the notion of an analogy between the phenomena of light and sound when he says: "The waves, pulses, or vibrations of the air, wherein sound consists, are manifestly inflected, though not so considerably as the waves of water; and sounds are propagated with equal case through crooked tubes and through straight lines; but light was never known to move in any curve, nor to inflect itself ad umbram." This decided statement is directly contradicted by actual experiment, because light can be bent into or towards the shadow. The corpuscular theory fails to explain that which is easily understood by the undulatory theory, and by analogy to waves of water or air, that two rays of light may come together in a special manner and produce darkness, just as two waves of water may interfere with each other and form a smooth surface, or two waves of sound produce silence. Dismissing the theory of Newton as we might pass by the venerable ruins of some ancient edifice, with mingled interest and regret, we may return to the consideration of the ether supposed to fill all space. The great Dr. Franklin, in a letter dated 23rd April, 1752, throws out the suggestion that all the phenomena of light may be more conveniently solved by supposing universal space filled with a subtile elastic fluid, which when at rest is not visible, but whose vibrations affect that fine sense in the eye as those of air do the grosser organs of the ear. Thornbury, Mitchell, and others, endeavoured to prove the materiality of light by showing that the corpuscles had a power of momentum which might affect other and very light substances. Could this fact have been really ascertained, there would be nothing more to say against Newton's hypothesis; but their experiments were illusory and useless. On the other hand, the supporters of the undulatory theory have within the last three years performed the most elaborate and exact experiments to try to prove the real existence of the ether. Mr. Balfour Stewart, F.R.S., superintendent of Kew Observatory, and Professor P. G. Tait, M.A., of Edinburgh, whilst leaving other scientific men to make their own deductions from the results they obtained, have called attention to the subject by a paper read before the Royal Society in June, 1865, and modestly entitled "On the Heating of a Disc by Rapid Rotation in vacuo.” The authors, having obtained certain results in air, were encouraged to construct the apparatus as figured below, Fig. 1, wherewith to procure rotation in vacuo. "In this apparatus a slowly revolving shaft is carried up through a barometer tube, having at its top the receiver which is to be exhausted. When the exhaustion has taken place, the shaft connected with the multiplying gear revolves in mercury. The train of toothed wheels causes the disc of alumi nium to revolve 125 times for each revolution of the shaft. The thermo-electric pile, the most delicate thermometer or test of heat, is connected by two wires carried through two holes in the bed-plate of the receiver with a Thompson's reflecting galvanometer needle (an instrument which is described and figured a, Figs. 1 and 2, thermo-electric pile with reflecting cone attached; at, height 6 in. from bed-plate; a length of cone, &c., 5 in.; ed, diameter of the aperture of the reflecting cone 24 in.; f, the disc of aluminum 13 in. diameter; eg, height from bed-plate to centre of the aluminum disc 8 in.; be, distance of centre of the thermo-electric pile from the disc of aluminum 8 in.; m, base containing the multiplying gear; sss, air-tight glass receiver, 15 in. diameter and 16 in. high, covering the whole. |