In 1891 he instituted a programme of wide-field photography for the discovery of asteroids, which appeared on the developed plates as trails among the stars. The first to be found in this way was (323) Brucia, in 1891. In all Wolf discovered over 200 asteroids; they include Achilles, the first Trojan asteroid to be found; in 1906. Independently of E. E. Barnard, Wolf proposed that ‘voids’ in the Milky Way are dark nebulae. He also discovered the North America Nebula (and gave it that name), Comet 14P/Wolf (1884), and various clusters of galaxies, including the Coma Cluster.