Along with the growth of observational astronomy which has taken place since the discoveries of Galileo, there has been developed a view concerning the physical history of the stellar world, known as the nebular hypothesis, which, though not yet fully proved, is believed by most astronomers and physicists to give us a tolerably correct notion as to the way in which the heavenly spheres were formed from an earlier condition of matter. This majestic conception was first advanced, in modern times at least, by the German philosopher Immanuel Kant. It was developed by the French astronomer Laplace, and is often known by his name. The essence of this view rests upon the fact previously noted that in the realm of the fixed stars there are many faintly shining aggregations of matter which are evidently not solid after the manner of the bodies in our solar system, but are in the state where their substances are in the condition of dustlike particles, as are the bits of carbon in flame or the elements which compose the atmosphere. The view held by Laplace was to the effect that not only our own solar system, but the centres of all the other similar systems, the fixed stars, were originally in this gaseous state, the material being disseminated throughout all parts of the heavenly realm, or at least in that portion of the universe of which we are permitted to know something. In this ancient state of matter we have to suppose that the particles of it were more separated from each other than are the atoms of the atmospheric gases in the most perfect vacuum which we can produce with the air-pump. Still we have to suppose that each of these particles attract the other in the gravitative way, as in the present state of the universe they inevitably do. Under the influence of the gravitative attraction the materials of this realm of vapour inevitably tended to fall in toward the centre. If the process had been perfectly simple, the result would have been the formation of one vast mass, including all the matter which was in the original body. In some way, no one has yet been able to make a reasonable suggestion of just how, there were developed in the process of concentration a great many separate centres of aggregation, each of which became the beginning of a solar system. The student may form some idea of how readily local centres may be produced in materials disseminated in the vaporous state by watching how fog or the thin, even misty clouds of the sunrise often gather into the separate shapes which make what we term a "mackerel" sky. It is difficult to imagine what makes centres of attraction, but we readily perceive by this instance how they might have occurred.