Laplace’s theory was powerfully supported by Herschel’s observations of the various nebulæ in the heavens. But, with the supposed resolution of the various nebulæ after the erection of the Rosse reflector in 1845, the evidence in favour of the nebular theory seemed to be greatly reduced. In 1864, however, the discovery of the gaseous nebulæ, by means of the spectroscope, gave further support to the theory. Powerful aid was lent to the nebular hypothesis by the famous German physicist, _Hermann Ludwig Ferdinand von Helmholtz_ (1821-1894), in 1854, in his theory of the maintenance of the Sun’s heat. Many theories had been already advanced to account for this. After the discovery of the conservation of energy, _Julius Robert Mayer_, one of the discoverers, put forward the theory that the Solar heat was sustained by the inflow of meteorites from space, and this idea was developed in 1854 by Sir _William Thomson_, now _Lord Kelvin_ (born 1824), but it was soon apparent that the supply of meteors required to sustain the Solar heat was such as would have increased the mass of the Sun very considerably. Accordingly the hypothesis was partially abandoned, and was succeeded by that of Helmholtz, who pointed out that the radiation of the Sun’s heat was the result of its contraction through cooling. The rate was then estimated at 380 feet yearly, or a second of arc in 6000 years. This theory was at once generally accepted. It assumes the Sun to be still contracting, and therefore, on going backwards in imagination, we reach a period when the Sun must have been much larger than now, and, in fact, extended beyond the orbit of Neptune. Several objections to Laplace’s nebular theory were urged by various investigators. Among these was the retrograde motions of the satellites of Uranus and Neptune, and the extremely rapid revolution of the inner satellite of Mars. Other objections were urged by Babinet, Kirkwood, and others, and at length a sweeping reform of the nebular theory was proposed by Faye in 1884, in his work, ‘Sur l’Origine du Monde.’ Faye put forward the idea that all the planets interior to the orbit of Uranus were formed inside the solar nebula, while Uranus and Neptune came into existence after the development of the Sun was far advanced. But the objections to Faye’s theory are formidable, and the hypothesis has not been accepted. A popular exposition of the nebular theory was given in 1901 in Ball’s work on ‘The Earth’s Beginning.’ He exhaustively discusses the whole question, and explains the retrograde motion of the satellites of Uranus and Neptune as due to the fact that the planes of the orbits of the satellites will eventually be brought to coincide with the ecliptic. These motions, says Ball, do not disprove the nebular theory. “They rather illustrate the fact that the great evolution which has wrought the Solar System into its present form has not finished its work: it is still in progress.”