Printed Letter, completed in manuscript, signed ‘Rayleigh’ as Secretary of the Royal Society, to Sir Benjamin Baker, sending a Paper, “Mr Wm. Brennand, On the Three-bar Motion of Watt”. 1 page 8 x 5 inches. The Royal Society, 21 January 1893. John William Strutt, 3rd Baron Rayleigh (1842-1919), physical scientist who made fundamental discoveries in the fields of acoustics and optics that are basic to the theory of wave propagation in fluids. He received the Nobel Prize for Physics in 1904 for his successful isolation of argon, an inert atmospheric gas. Rayleigh shared the priority of the discovery with the chemist William Ramsay, who also isolated the new gas, though he began his work after Rayleigh’s publication of the original density discrepancy. Perhaps Rayleigh’s most significant early work was his theory explaining the blue colour of the sky as the result of scattering of sunlight by small particles in the atmosphere. The Rayleigh scattering law, which evolved from this theory, has since become classic in the study of all kinds of wave propagation.