splendid theory which seems to have been one of the flashes of Herschel's genius".
The light of the stars, no less than their motions, was a favourite subject of study with Herschel. His first communication to the Royal Society dealt with the famous variable star Mira Ceti; and several other papers dealt with the brilliancy of the stars. The problem of stellar variation fascinated him. "Dark spots, or large portions of the surface less luminous than the rest, turned alternately in certain directions, either towards or from us," he wrote in 1796, "will account for all the phenomena of periodical changes in the lustre of the stars so satisfactorily that we certainly need not look for any other cause." But he was aware of other variations—the gradual increase or decrease of the light of certain stars in the course of years or centuries. He regarded as a problem of great practical interest the stability or otherwise of the brilliance of the Sun. His interest in the question of stellar brightness and variation led to his determination of the relative brightness of the stars. Four catalogues of comparative brightness of stars were communicated to the Royal Society—the first on 25th February, 1796, the fourth on 21st February, 1799. The observations on which these catalogues were based were made in the years 1795-97, and Herschel seems to have attached so much importance to the work that for three years he discontinued his "sweeping" of the heavens. Two other catalogues were left unpublished at the time of his death, and were first issued in the collected edition of his works. The observations he himself described as "difficult and laborious". He mentioned in the paper accompanying his first catalogue the various causes of error which had to be guarded against, such as moonlight, the different altitudes at which a star might be viewed, the uncertainty of flying cloud, the scintillation of the stars, the zodiacal light, the aurora borealis, and "dew or damp upon the glasses or specula when a
