RAYLEIGH, JOHN WILLIAM STRUTT, 3RD BARON (1842-1919), English physicist, was born in Essex on Nov. 12, 1842, and educated at Trinity college, Cambridge. He succeeded to his father's title in 1873. From 1879 to 1884 he was Cavendish professor of experimental physics in the University of Cambridge, and from 1887 to 1905, professor of natural philosophy at the Royal Institution of Great Britain. In 1896 he became scientific adviser to Trinity House, and in 1901 chief gas examiner. His early papers were mainly mathematical, including two on electro magnetic phenomena considered in connection with dynamical theory, and one on the stationary thermal conditions of a sphere exposed to radiation from distant sources. His paper on reso nance was the first of a series of memoirs which culminated in his standard work, A Treatise on Sound. Other papers on sound dealt with the vibrations of a gas contained in a rigid spherical envelope, disturbances produced in sound waves by a spherical obstacle, general theorems relating to vibrations, absolute pitch and the re sultant of a large number of vibrations of the same pitch and of arbitrary phase. He carried out experiments on the amplitude of the sound wave near the limit of audibility and on the direction of sound. Rayleigh also carried out investigations in optics and ex periments in colour vision. He wrote on polarization, the scatter ing of light by particles, and the colour of the sky. Great ex perimental skill was shown in his reproductions of gratings by photographic methods. A series of four papers entitled "Investiga tions in Optics, with special reference to the Spectroscope," were published in the Philosophical Magazine in 1879 and 1880. These dealt with the general question of the resolving power of optical instruments. Later he dealt more particularly with the microscope.
His contributions to hydrodynamics were important ; he dealt with problems connected with the propagation of waves, showing how the wave velocity depends on the period. He applied this to
the case of deep sea water waves and showed the part played by waves in earthquakes. Rayleigh's experiments on the instability of water jets led him to work on surface tension. While at the Cavendish laboratory he carried out his determination of the ohm.
Rayleigh also worked out a formula for the distribution of energy in black-body radiation which holds for long wave-lengths. His interest in Prout's hypothesis as to atomic weights led him to a series of experiments to determine the densities of gases. While working on nitrogen Rayleigh made observations which led to the discovery of argon. Sir William Ramsay co-operated with him in the latter stages of the work. Lord Rayleigh had an interest in abnormal psychological investigations, and became a member and vice-president of the Society for Psychical Research. He was one of the original members of the Order of Merit, instituted in connection with the coronation of King Edward VII. In 1904 he was awarded a Nobel prize, and at the end of 1905 he became president of the Royal Society, of which he had been elected a fel low in 1873, and had acted as secretary from 1885 to 1896. He remained president till 1908, in which year he succeeded the 8th duke of Devonshire as chancellor of Cambridge university. In 1909 the prime minister, Asquith, set up a committee of aero nautics, of which Rayleigh was president. Rayleigh was the re cipient of many academic and other honours. He died at Witham, Essex, on June 3o, 1919.
Rayleigh's papers are collected in four volumes. An account of his life, by his son, was published in 1924. See obituary notice by Sir A. Schuster in Proceedings of the Royal Society (vol. 98, 1920-21).