John Herschel’s favourite objects of study were double stars, of which he discovered 3347 in the northern hemisphere, and 1202 in the southern. He also computed several stellar orbits; but the first calculation of a stellar orbit was made by the French astronomer _Felix Savary_ (1797-1841), who computed the orbit of ξ Ursæ Majoris, and found the period to be about sixty years. Contemporary with John Herschel was his great rival in double-star astronomy, Friedrich Georg Wilhelm Struve. Born at Altona in 1793, Struve took his degree in 1811 at the Russian University of Dorpat. In 1813 he became director of the Dorpat Observatory, and was in 1839 promoted to Pulkowa, as director of the great Observatory there, remaining at its head until within three years of his death, on November 23, 1864. Struve’s first recorded observation was on the double star Castor. In 1819 he commenced to measure the position-angles of double stars, of which he published a catalogue of 795. In 1825 he commenced a review of the heavens down to fifteen degrees south, and thus discovered 2200 previously unknown objects. The results were published in Struve’s ‘Mensuræ Merometricæ,’ which appeared in 1836, giving the places, distances, colours, position-angles, and relative brilliance of 3112 double and multiple stars. Struve’s successor in this branch of astronomy was his son, Otto Wilhelm von Struve, born in 1819 at Dorpat, who became in 1837 assistant to his father, and in 1861 succeeded him as director of the Pulkowa Observatory. In 1890 he retired from this post, settling in Germany, at Carlsruhe, where, on April 14, 1905, he died in his eighty-sixth year. Otto Struve detected 500 double stars, among them γ Andromedæ, discovered in 1842, and δ Equulei, discovered in 1852, within a period of between five and eleven years.