The nets spread in his “sweeps” brought in, besides double stars, plentiful takes of the filmy objects called “nebulæ.” He recognised with amazement their profusion in certain tracts of the sky; increased telescopic light-grasp never failed to render a further supply visible; the heavens teemed with them. He presented a catalogue of 1,000 to the Royal Society in 1786, a second equally comprehensive in 1789, and a supplementary list of 500 in 1802. Their natural history fascinated him. What they were, what they had been, and what they should come to, formed the subject of many of those ardent meditations which supplied motive power for his researches. He not only laid the foundation of nebular science, but carried the edifice to a considerable height, distinguishing the varieties of its objects, and classifying them according to their gradations of brightness. Some presented a most fantastic appearance.
“I have seen,” he wrote in 1784, “double and treble nebulæ variously arranged; large ones with small, seeming attendants; narrow, but much extended lucid nebulæ or bright dashes; some of the shape of a fan, resembling an electric brush, issuing from a lucid point; others of the cometic shape, with a seeming nucleus in the centre, or like cloudy stars surrounded with a nebulous atmosphere; a different sort, again, contained a nebulosity of the milky kind, like that wonderful, inexplicable phenomenon about Theta Orionis; while others shine with a fainter mottled kind of light which denotes their being resolvable into stars.”
He, “through the mystic dome,” discerned
“Regions of lucid matter taking form,
Brushes of fire, hazy gleams,
Clusters and beds of worlds, and bee-like swarms
Of suns and starry streams.”
Annular and planetary nebulæ were as such, first described by him. “Among the curiosities of the heavens,” he announced in 1785, “should be placed a nebula that has a regular concentric dark spot in the middle, and is probably a ring of stars.” This was the famous annular nebula in Lyra, then a unique specimen, now the type of a class.
The planetary kind, so-called from their planet-like discs, were always more or less of an enigma to him. The vividness and uniformity of their light appeared to cut them off from true nebulæ; on mature consideration, he felt driven to suppose them “compressed star-groups.” “If it were not, perhaps, too hazardous,” he went on, “to pursue a former surmise of a renewal in what I figuratively called the laboratories of the universe, the stars forming these extraordinary nebulæ, by some decay or waste of nature, being no longer fit for their former purposes, and having their projectile forces, if any such they had, retarded in each other’s atmospheres, rush at last together, and either in succession, or by one general tremendous shock, unite into a new body. Perhaps the extraordinary and sudden blaze of a new star in Cassiopeia’s Chair, in 1572, might possibly be of such a nature.”
At that early stage of his inquiries, Herschel regarded all nebulæ indiscriminately as composed of genuine stars. It was almost inevitable that he should do so. For each gain in telescopic power had the effect of transferring no insignificant proportion of them from the nebular to the stellar order. There was no apparent reason for drawing a line anywhere. The inference seemed irresistible, that resolvability was simply a question of optical improvement. As Messier’s nébuleuses sans étoiles had yielded to Herschel’s telescopes, so—it might fairly be anticipated—the “milky” streaks and patches seen by Herschel would curdle into stars under the compulsion of the still mightier instruments of the future. He was led on—to use his own expressions in 1791—“by almost imperceptible degrees from evident clusters, such as the Pleiades, to spots without a trace of stellar formation, the gradations being so well connected as to leave no doubt that all these phenomena were equally stellar.” They were what Lambert and Kant had supposed them to be—island-universes, vast congeries of suns, independently organised, and of galactic rank. They were, each and all, glorious systems, barely escaping total submergence in the illimitable ocean of space. Under the influence of these grandiose ideas, Herschel told Miss Burney, in 1786, that with his “large twenty-foot” he had “discovered 1,500 universes!” Fifteen hundred “whole sidereal systems, some of which might well outvie our Milky Way in grandeur.”