[Sidenote: The modern theory of light was established only about the year 1830.]
The nature of light has been in every age a fascinating subject for study and reflection. Descartes, the French mathematician and philosopher, advanced the hypothesis that light consists of small particles emitted by luminous bodies, and that the sensation of light is produced by the impact of these particles upon the retina of the eye. Soon after this emission or corpuscular theory had been proposed, Hooke, an English investigator of great note, stated publicly that the phenomena of light, as he had observed them, led him to the belief that the nature of light could best be explained on the assumption that light was a kind of undulation or wave in some unknown medium, and that the sensation of light was. produced when these waves struck upon the retina of the eye. This new hypothesis, known as the theory of undulations, after the great Isaac Newton had declared himself in favor of the corpuscular theory, was finally adjudged by the majority of students to be erroneous.
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.]
The nature of heat had long been discussed when the world of science decided in favor of the undulatory theory of light. One school held that the sensation of heat was caused by the cannonading of heat particles by the heated body; the other school, with few adherents, insisted that heat was simply a form of motion of the ether already adopted in the theory of light. The later discoveries of science proved with considerable certainty that the undulatory theory of heat is right, but it was well towards the middle of the last century before the emission theory of heat lost its ground. In fact, Dr.Whewell, in the third edition of his classic book on the History of Inductive Sciences, published in 1859, says that the undulatory theory of heat "has not by any means received full confirmation;"[A] and Dr. John Tyndall, in a book published in 1880, says, that the emission theory "held its ground until quite recently among the chemists of our own day."[B] Today, the evidences of modern science are overwhelmingly in favor of the undulatory theory of heat.