CHAPTER XI. 197

STELLAR SYSTEMS AND NEBULÆ. Life and work of John Herschel—Binary stars—Computation of orbits—Work of Wilhelm Struve—Of Otto Struve—Of Burnham—Satellites of Sirius and Procyon—Astronomy of the invisible—Work of Pickering and Vogel—Spectroscopic binaries—Work of Bélopolsky and Campbell—Star-clusters—Nature of nebulæ—Spectroscopic work of Huggins—Of Copeland—Nebular photography—Work of Roberts, Barnard, Wolf—Of Keeler