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 beginning with red and ending in white. The testimony of practical astronomers was soon given that such appearances are not observed; and the hypothesis was accordingly abandoned.

The fortunes of the wave-theory began to brighten at the end of the century, when a new champion arose. Thomas Young, born at Milverton in Somersetshire in 1773, and trained to the practice of medicine, began to write on optical theory in 1799. In his first paper he remarked that, according? to the corpuscular theory, the velocity of emission of a corpuscle must be the same in all cases, whether the projecting force be that of the feeble spark produced by the friction of two pebbles, or the intense heat of the sun itself—a thing almost incredible. This difficulty does not exist in the undulatory theory, since all disturbances are known to be transmitted) through an elastic fluid with the same velocity. The reluctance which some philosophers felt to filling all space with an elastic fluid he met with an argument which strangely foreshadows the electric theory of light: "That a medium resembling in many properties that which has been denominated ether does really exist, is undeniably proved by the phenomena of electricity. The rapid transmission of the electrical shock shows that the electric medium is possessed of an elasticity as great as is necessary to be supposed for the propagation of light. Whether the electric ether is to be considered the same with the luminons ether, if such a fluid exists, may perhaps at some future time be discovered by experiment: hitherto I have not been able to observe that the refractive power of a fluid, undergoes any change by electricity."

Young then proceeds to show the superior power of the wave-theory to explain reflexion and refraction, In the corpuscular theory it is difficult to see why part of the light should be reflected and another part of the same beam reflected; but in the undulatory theory there is no trouble, as is shown by analogy with the partial reflexion of sound from a cloud or denser stratum of air: "Nothing more is necessary than to