These preliminary attempts shrank into insignificance when Herschel began to "sweep the heavens" with his giant telescopes. In 1786 he presented to the Royal Society a descriptive catalogue of 1,000 nebulæ and clusters, followed, three years later, by a second of as many more; to which he added in 1802 a further gleaning of 500. On the subject of their nature his views underwent a remarkable change. Finding that his potent instruments resolved into stars many nebulous patches in which no signs of such a structure had previously been discernible, he naturally concluded that "resolvability" was merely a question of distance and telescopic power. He was (as he said himself) led on 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. The singular variety of their appearance was thus described by him:—
"I have seen," he says, "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, contain a nebulosity of the milky kind, like that wonderful, inexplicable phenomenon about θ Orionis; while others shine with a fainter, mottled kind of light, which denotes their being resolvable into stars."[46]
"These curious objects" he considered to be "no less than whole sidereal systems,"[47] some of which might "well outvie our Milky Way in grandeur." He admitted, however, a wide diversity in condition as well as compass. The system to which our sun belongs he described as "a very extensive branching congeries of many millions of stars, which probably owes its origin to many remarkably large as well as pretty closely scattered small stars, that may have drawn together the rest."[48] But the continued action of this same "clustering power" would, he supposed, eventually lead to the breaking-up of the original majestic Galaxy into two or three hundred separate groups, already visibly gathering. Such minor nebulæ, due to the "decay" of other "branching nebulæ" similar to our own, he recognised by the score, lying, as it were, stratified in certain quarters of the sky. "One of these nebulous beds," he informs us, "is so rich that in passing through a section of it, in the time of only thirty-six minutes, I detected no less than thirty-one nebulæ, all distinctly visible upon a fine blue sky." The stratum of Coma Berenices he judged to be the nearest to our system of such layers; nor did the marked aggregation of nebulæ towards both poles of the circle of the Milky Way escape his notice.
By a continuation of the same process of reasoning, he was enabled (as he thought) to trace the life-history of nebulæ from a primitive loose and extended formation, through clusters of gradually increasing compression, down to the kind named by him "Planetary" because of the defined and uniform discs which they present. These he regarded as "very aged, and drawing on towards a period of change or dissolution."[49]
"This method of viewing the heavens," he concluded, "seems to throw them into a new kind of light. They now are seen to resemble a luxuriant garden which contains the greatest variety of productions in different flourishing beds; and one advantage we may at least reap from it is, that we can, as it were, extend the range of our experience to an immense duration. For, to continue the simile which I have borrowed from the vegetable kingdom, is it not almost the same thing whether we live successively to witness the germination, blooming, foliage, fecundity, fading, withering, and corruption of a plant, or whether a vast number of specimens, selected from every stage through which the plant passes in the course of its existence, be brought at once to our view?"[50]
But already this supposed continuity was broken. After mature deliberation on the phenomena presented by nebulous stars, Herschel was induced, in 1791, to modify essentially his original opinion.
"When I pursued these researches," he says, "I was in the situation of a natural philosopher who follows the various species of animals and insects from the height of their perfection down to the lowest ebb of life; when, arriving at the vegetable kingdom, he can scarcely point out to us the precise boundary where the animal ceases and the plant begins; and may even go so far as to suspect them not to be essentially different. But, recollecting himself, he compares, for instance, one of the human species to a tree, and all doubt upon the subject vanishes before him. In the same manner we pass through gentle steps from a coarse cluster of stars, such as the Pleiades ... till we find ourselves brought to an object such as the nebula in Orion, where we are still inclined to remain in the once adopted idea of stars exceedingly remote and inconceivably crowded, as being the occasion of that remarkable appearance. It seems, therefore, to require a more dissimilar object to set us right again. A glance like that of the naturalist, who casts his eye from the perfect animal to the perfect vegetable, is wanting to remove the veil from the mind of the astronomer. The object I have mentioned above is the phenomenon that was wanting for this purpose. View, for instance, the 19th cluster of my 6th class, and afterwards cast your eye on this cloudy star, and the result will be no less decisive than that of the naturalist we have alluded to. Our judgment, I may venture to say, will be, that the nebulosity about the star is not of a starry nature."[51]
The conviction thus arrived at of the existence in space of a widely diffused "shining fluid" (a conviction long afterwards fully justified by the spectroscope) led him into a field of endless speculation. What was its nature? Should it "be compared to the coruscation of the electric fluid in the aurora borealis? or to the more magnificent cone of the zodiacal light?" Above all, what was its function in the cosmos? And on this point he already gave a hint of the direction in which his mind was moving by the remark that this self-luminous matter seemed "more fit to produce a star by its condensation, than to depend on the star for its existence."[52]
This was not a novel idea. Tycho Brahe had tried to explain the blaze of the star of 1572 as due to a sudden concentration of nebulous material in the Milky Way, even pointing out the space left dark and void by the withdrawal of the luminous stuff; and Kepler, theorising on a similar stellar apparition in 1604, followed nearly in the same track. But under Herschel's treatment the nebular origin of stars first acquired the consistency of a formal theory. He meditated upon it long and earnestly, and in two elaborate treatises, published respectively in 1811 and 1814, he at length set forth the arguments in its favour. These rested entirely upon the "principle of continuity." Between the successive classes of his assortment of developing objects there was, as he said, "perhaps not so much difference as 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."[53] From diffused nebulosity, barely visible in the most powerful light-gathering instruments, but which he estimated to cover nearly 152 square degrees of the heavens,[54] to planetary nebulæ, supposed to be already centrally solid, instances were alleged of every stage and phase of condensation. The validity of his reasoning, however, was evidently impaired by his confessed inability to distinguish between the dim rays of remote clusters and the milky light of true gaseous nebulæ.
It may be said that such speculations are futile in themselves, and necessarily barren of results. But they gratify an inherent tendency of the human mind, and, if pursued in a becoming spirit, should be neither reproved nor disdained. Herschel's theory still holds the field, the testimony of recent discoveries with regard to it having proved strongly confirmatory of its principle, although not of its details. Strangely enough, it seems to have been propounded in complete independence of Laplace's nebular hypothesis as to the origin of the solar system. Indeed, it dated, as we have seen, in its first inception, from 1791, while the French geometrician's view was not advanced until 1796.