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BOUT 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."