An idea of the vast importance of Keeler’s work may be gained if we reflect that the observations of all the earlier astronomers resulted in the discovery of six thousand nebulæ. The investigations of Keeler, in all probability, were the means of adding 120,000 more. Many observations have been made on nebulæ, for the purpose of ascertaining their proper motions—but without success. Measurements were made by D’Arrest in 1857 and by Burnham in 1891, but none of these revealed any motion of the nebulæ across the line of sight. Even the new spectroscopic method of determining motions in the line of sight, in the hands of Huggins, failed in the case of the nebulæ. With the great Lick refractor at his disposal, Keeler attacked the subject in 1890, and measured the radial velocities of ten nebulæ. He found that the well-known planetary nebula in Draco was moving towards the Solar System at the rate of 40 miles a second; for the Orion nebula he found a motion of recession of 11 miles a second; but probably this belongs chiefly to the movement of the Solar System in the opposite direction. Unfortunately Keeler did not live to carry on his investigations in nebular astronomy. His early death brought to an abrupt end these fruitful investigations. Appointed director of the Lick Observatory in 1898, he died suddenly at San Francisco on August 12, 1900, at the early age of forty-two. CHAPTER XII. STELLAR DISTRIBUTION AND THE STRUCTURE OF THE UNIVERSE. After the death of Herschel there was little done in the direction of furthering our knowledge of stellar distribution, or the construction of the heavens. Here, as elsewhere, Herschel’s immediate successor was his son, whose star-gauges, both in England and in South Africa, were a worthy sequel to those of his father; but John Herschel, in his books on astronomy, reproduced his father’s disc-theory, unaware that the elder Herschel had himself abandoned it. The work of the younger Herschel was entirely supplementary to that of his father.