gave evidence of age. Accordingly, he foresaw, as a result of "clustering power," the breaking-up of the galactic system into many small independent nebulæ. More and more evidences of this "clustering power" came to his notice until in 1802 he said of the Galaxy: "This immense starry aggregation is by no means uniform. The stars of which it is composed are very unequally scattered and show evident marks of clustering together into many separate allotments." He was coming gradually to the view that the fundamental assumption underlying his disc-theory that of an average equality of scattering was untenable. In his paper of 1811 he said: "I must freely confess that by continuing my sweeps of the heavens, my opinion of the arrangement of the stars and their magnitudes and of some other particulars has undergone a gradual change; and, indeed, when the novelty of the subject is considered, we cannot be surprised that many things formerly taken for granted should, on examination, prove to be different from what they were generally but incautiously supposed to be. For instance, an equal scattering of stars may be admitted in certain calculations; but when we examine the Milky Way, or the closely-compressed clusters of stars, of which my catalogues have recorded so many instances, this supposed equality must be given up." With the abandonment of this general assumption, the disc-theory became untenable.
Herschel's daring attempt to formulate a cosmology proved abortive. In place of this he was led to evolve a cosmogony. He appears to have been unaware of Kant's nebular hypothesis; indeed, he seems to have had, at the beginning of his career, no conception of evolutionary development among the celestial bodies. The dim, misty-looking nebulæ were all believed to be external galaxies, which increased telescopic power could resolve into their component stars. He was led to question and then to reject this generalisation by his study of a
