About the year 1800, more than a century after the days of Descartes, Hooke and Newton, an English physician, Dr. Thomas Young, who had long experimented on the nature of light, asserted that the emission theory could not explain many of the best known phenomena of light. Dr. Young further claimed that correct explanations could be made only by the theory of waves of undulation of an etherial medium diffused through space, and presented numerous experimental evidences in favor of this view. This revival of the old theory of undulation met at first with violent opposition from many of the greatest scientific minds of the day. Sometime after Dr. Young's publication, a French army officer, Augustine Fresnel, undertook the study of the nature of light, and arrived, almost independently, at the conclusion stated by Dr. Young. Later, other investigators discovered light phenomena which could be explained only on the undulatory hypothesis, and so, little by little, the new theory gained ground and adherents. Still, even as late as 1827, the astronomer Herschel published a treatise on light, in which he appeared to hold the real merit of the theory of undulations in grave doubt.[A] Likewise, the Imperial Academy at St. Petersburg, in 1826, proposed a prize for the best attempt to relieve the undulatory theory of light of some of the main objections against it.[B] It was several years later before the great majority of the scientific world accepted the theory of undulations as the correct explanation of the phenomena of light. [Footnote A: History of the Inductive Sciences, Whewell, 3rd edition, Vol. II, p. 114.] [Footnote B: Loc. cit., 117.] [Sidenote: A subtle substance, the ether, fills all space.] In brief, this theory assumes that a very attenuated, but very elastic, substance, called the ether, fills all space, and is found surrounding the ultimate particles of matter. Thus, the pores of wood, soil, lead, gold and the human body, are filled with the ether. It is quite impossible by any known process to obtain a portion of space free from it. A luminous body is one in which the ultimate particles of matter, the atoms or molecules, are moving very rapidly, and thus causing disturbances in the ether, similar to the disturbances in quiet water when a rock is thrown into it; and, like the water wave, proceeding from the point of disturbance, so the ether waves radiate from the luminous body into space. When a wave strikes the retina of the eye, the sensation of light is produced. This new-found ether was soon used for the explanation of other natural phenomena. [Sidenote: Light, heat, electricity and other forces are forms of ether motion.]