awakened public interest in the debatable land which lies between that which is certainly known to science and that which must always defy inquiry. Before the appearance of that remarkable book, the theory that the sun and its attendant planets were produced by the condensation of a vast nebula was but little known to the unscientific world. The idea was originally entertained by Sir William Herschel, and affords one of the greatest proofs of his commanding genius. It was afterward elaborated by Laplace; but that great astronomer was himself distrustful of it, and, while he expounded the mechanical laws by which the proposed explanation could be supported, he was careful to speak of it only as an hypothesis. As time goes on, it seems probable that the saying of Arago will be accepted, and that the views of Laplace will be universally acknowledged to be "those only which, by their grandeur, their coherence, and their methematical character, can be truly considered to form a physical cosmogony."
But, though Laplace is thus credited by Arago with the origination of this grand conception, he was not its author. Sir William Herschel gave the earliest sketch of the theory. His views w r ere expressed with so much precision, that one cannot help feeling a little jealousy for the prior right of discovery of the English astronomer. Herschel so plainly preceded Laplace, that it seems hard that Laplace should have the credit of it. Herschel began to search after nebulae in 1779, and soon formed a catalogue comprising an enormous number of them. By degrees it dawned upon his mind that the differences he observed in them were systematic, and at length occurred the magnificent intuition that the nebulæ are stars in process of formation.
They lie in enormous numbers in every part of the heavens, and apparently in every stage of progressive development. The slow growth of worlds, extending over ages of time, cannot, of course, be watched by any single observer. No more can a single tree among the trees of a forest be so observed. But a forest contains specimens of saplings, young trees, trees of vigorous growth, and trees in decay. In like manner the heavens contain specimens of worlds in the making, from the chaotic mass of vapory matter which forms the first stage of cosmical existence to the perfect, self-luminous star. Herschel arranged them in classes showing this gradual development, and he declares that each class is so nearly allied to the next, that they do not differ so much as would the annual description of a human figure, if it were given from the birth of a child till he comes to be a man in his prime. His catalogue arranges the objects he has actually observed somewhat in the following fashion: first, patches of extensive diffused nebulosity; "milky nebulosity," with condensation; round nebulæ; nebulæ with a nucleus; and soon till he reaches stellar nebulæ, nearly approaching the appearance of stars.
The evidence grows irresistible as we read, that in these wonder-
