PROF. DE SITTER, Director of Leyden Observatory, delivered the George Darwin Lecture of the Royal Astronomical Society on May 8, taking as his subject “The Satellites of Jupiter”. The lecture began with a sketch of the progress of our knowledge of the system. Galileo attempted to make tables of their motion. Romer deduced from them the finite velocity of light. War-gentin devoted a large part of his life to the study of their motions. Bradley made careful observations. La Grange improved the mathematical theory. Delambre and Damoiseau made tables, which remained in use until recent times. Forty years ago, Sir David Gill carried out a series of observations with the Cape heliometer he compared the satellites with each other, not with Jupiter itself, finding that this increased the accuracy of observation very notably. The positions of certain stars had been found with great accuracy in connexion with the determination of the solar parallax from observations of the planets Iris, Victoria, and Sappho. These stars were now used to check the scale of the heliometer, and this was considered to be known to one part in 100,000. Prof, de Sitter took a large part in reducing these observations he found from a combination of all determinations that the mass of the Jovian system is 1/1047.40 of the sun, with a probable error of 0.03 in the denominator.