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 Johns Hopkins University in 1884. Neither this nor the first-named school succeeded in explaining all the phenomena. A third school was founded by Maxwell. He proposed the electro-magnetic theory, which has received extensive development recently. It will be mentioned again later. According to Maxwell's theory, the direction of vibration does not lie exclusively in the plane of polarisation, nor in a plane perpendicular to it, but something occurs in both planes—a magnetic vibration in one, and an electric in the other. Fitzgerald and Trouton in Dublin verified this conclusion of Maxwell by experiments on electro-magnetic waves.

Of recent mathematical and experimental contributions to optics, mention must be made of H. A. Rowland's theory of concave gratings, and of A. A. Michelson's work on interference, and his application of interference methods to astronomical measurements.

In electricity the mathematical theory and the measurements of Henry Cavendish (1731–1810), and in magnetism the measurements of Charles Augustin Coulomb (1736–1806), became the foundations for a system of measurement. For electro-magnetism the same thing was done by Andrè Marie Ampère (1775–1836). The first complete method of measurement was the system of absolute measurements of terrestrial magnetism introduced by Gauss and Wilhelm Weber (1804–1891) and afterwards extended by Wilhelm Weber and F. Kohlrausch to electro-magnetism and electro-statics. In 1861 the British Association and the Royal Society appointed a special commission with Sir William Thomson at the head, to consider the unit of electrical resistance. The commission recommended a unit in principle like W. Weber's, but greater than Weber's by a factor of $\scriptstyle{10^7}$.[101] The discussions and labours on this subject continued for twenty years, until in 1881 a general agreement was reached at an electrical congress in Paris.