Copy letter, from Sir John Herschel to Stephen Peter Rigaud, dated at Slough Copy letter in John Herschel's hand. Has correlated James Bradley's observations of the Castor and the Gamma Virginis double stars systems, which observations Stephen Rigaud sent to John Frederick William Herschel, with later observations. Agreement is excellent. John Frederick William Herschel lays out elements of orbits for each system. 15 August 1831