Campbell finds that of 285 stars observed by him, more than one in nine is a spectroscopic binary. He concludes that at least one star in five or six will be found to be spectroscopically double, and considers that “the proven existence of so large a number of stellar systems, differing so widely in structure from the Solar System, gives rise to a suspicion at least that our system is not of the prevailing type of stellar systems.” The study of triple and multiple stars is of deep interest, but the orbits of these objects cannot be said to be fully investigated by any means. The first application of the problem of three bodies to stellar astronomy was made by Seeliger in 1889. His researches, relating to the famous star, ζ Cancri, disclosed the existence of three stars revolving round a dark body, apparently the most massive in the system. The system of ζ Cancri, at least, seems to be modelled on the Ptolemaic design. In the study of star-clusters and nebulæ, as in the investigation of double stars, Herschel’s successor was his son. His observations, both in England and at the Cape of Good Hope, resulted in a large number of new discoveries, and the results of his studies in this direction were published in 1864 in his catalogue of all known clusters and nebulæ, amounting to 5079. This catalogue was enlarged and revised in 1888 by _John Louis Emil Dreyer_ (born 1852), a Danish astronomer, but director of the Observatory at Armagh, in Ireland; and the same observer published from 1888 to 1894 a supplementary list, bringing the number of known clusters and nebulæ to about 10,000. In the early part of his career, John Herschel held firmly to the views of his father of the difference between star-clusters and nebulæ, considering the latter to be composed of “shining fluid.” But he fell off from this view with the resolution into stars of many irresolvable nebulæ. In 1845 _William Parsons_, third _Earl of Rosse_ (1800-1867), erected at Birr Castle, in Ireland, his great 6-foot reflector, which still surpasses all other telescopes in point of size. With this instrument Lord Rosse believed himself to have resolved the Crab nebula in Taurus and the Nebula in Orion, which was also said to have been resolved by Bond with the 15-inch refractor at Harvard; and in 1854 Olmsted declared the “resolution” of these nebulæ to be the signal for the renunciation of Herschel’s nebular theory. Most astronomers fell in with the view that all the nebulæ were distant clusters, which would eventually be resolved into stars, although it is only right to state that the Scottish astronomer, _John Pringle Nichol_ (1804-1859), and some other investigators, held to the theory of Herschel.