1811, in another epoch-making paper on the construction of the heavens, that Herschel enunciated his nebular hypothesis. In this paper he gave a complete list of nebulæ which he had discovered and studied, "assorting them into as many classes as will be required to produce the most gradual affinity between the individuals contained in any one class with those contained in that which precedes and that which follows it". Those contained in one class and those in the next class in order, he declared, have not so much difference between them, in his own suggestive remark, "as there would be in an annual description of the human figure, were it given from the birth of a child till he comes to be a man in his prime". He traced the evolutionary sequence from extensive diffused nebulosities, through irregular nebulæ, "nebulæ a little brighter in the middle," "nebulæ a little brighter" and "much brighter in the middle," nebulæ showing the progress of condensation, planetary nebulæ and stellar nebulæ, to "nebulæ nearly approaching to the appearance of stars". He declared it highly probable that "every succeeding state of the nebulous matter is the result of the action of gravitation upon it while in a foregoing one, and by such steps the successive condensation of it has been brought up to the planetary condition. From this the transit to the stellar form, it has been shown, requires but a very small additional compression of the nebulous matter." In 1814 he drew attention to double nebulæ joined by nebulosity between them. "It seems," he said, "as if we had these double objects in three different successive conditions: first as nebulæ; next as stars with remaining nebulosity; and lastly as stars completely free from nebulous appearance". Herschel's nebular hypothesis has never received in text-books of astronomy the attention it deserves. It was the result of long years of patient study, and is one of the most perfect examples of inductive reasoning in the history of science.
