"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 pa.s.ses 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 pa.s.s through gentle steps from a coa.r.s.e cl.u.s.ter 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 cl.u.s.ter of my 6th cla.s.s, 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 s.p.a.ce 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 s.p.a.ce 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 cla.s.ses of his a.s.sortment 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 nebulae, 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 cl.u.s.ters and the milky light of true gaseous nebulae.

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.

We may now briefly sum up the chief results of Herschel's long years of "watching the heavens." The apparent motions of the stars had been disentangled; one portion being clearly shown to be due to a translation towards a point in the constellation Hercules of the sun and his attendant planets; while a large balance of displacement was left to be accounted for by real movements, various in extent and direction, of the stars themselves. By the action of a central force similar to, if not identical with, gravity, suns of every degree of size and splendour, and sometimes brilliantly contrasted in colour, were seen to be held together in systems, consisting of two, three, four, even six members, whose revolutions exhibited a wide range of variety both in period and in orbital form. A new department of physical astronomy was thus created,[55] and rigid calculation for the first time made possible within the astral region. The vast problem of the arrangement and relations of the millions of stars forming the Milky Way was shown to be capable of experimental treatment, and of at least partial solution, notwithstanding the variety and complexity seen to prevail, to an extent previously undreamt of, in the arrangement of that majestic system. The existence of a luminous fluid, diffused through enormous tracts of s.p.a.ce, and intimately a.s.sociated with stellar bodies, was virtually demonstrated, and its place and use in creation attempted to be divined by a bold but plausible conjecture. Change on a stupendous scale was inferred or observed to be everywhere in progress. Periodical stars shone out and again decayed; progressive ebbings or flowings of light were indicated as probable in many stars under no formal suspicion of variability; forces were everywhere perceived to be at work, by which the very structure of the heavens themselves must be slowly but fundamentally modified. In all directions groups were seen to be formed or forming; tides and streams of suns to be setting towards powerful centres of attraction; new systems to be in process of formation, while effete ones hastened to decay or regeneration when the course appointed for them by Infinite Wisdom was run. And thus, to quote the words of the observer who "had looked farther into s.p.a.ce than ever human being did before him,"[56] the state into which the incessant action of the cl.u.s.tering power has brought the Milky Way at present, is a kind of chronometer that may be used to measure the time of its past and future existence; and although we do not know the rate of going of this mysterious chronometer, it is nevertheless certain that, since the breaking-up of the parts of the Milky Way affords a proof that it cannot last for ever, it equally bears witness that its past duration cannot be admitted to be infinite.[57]

FOOTNOTES:

[Footnote 3: _Phil. Trans._, vol. x.x.x., p. 737.]

[Footnote 4: Out of eighty stars compared, fifty-seven were found to have changed their places by more than 10". Lesser discrepancies were at that time regarded as falling within the limits of observational error.

_Tobiae Mayeri Op. Inedita_, t. i., pp. 80, 81, and Herschel in _Phil.

Trans._, vol. lxxiii., pp. 275-278.]

[Footnote 5: _Posthumous Works_, p. 701.]

[Footnote 6: Arago in _Annuaire du Bureau des Longitudes_, 1842, p.

313.]

[Footnote 7: Bradley to Halley, _Phil. Trans._, vol. x.x.xv. (1728), p.

660. His observations were directly applicable to only two stars, Gamma Draconis and Eta Ursae Majoris, but some lesser ones were included in the same result.]

[Footnote 8: Holden, _Sir William Herschel, his Life and Works_, p. 17.]

[Footnote 9: _Phil. Trans._, vol. ci., p. 269.]

[Footnote 10: Caroline Lucretia Herschel, born at Hanover, March 16, 1750, died in the same place, January 9, 1848. She came to England in 1772, and was her brother's devoted a.s.sistant, first in his musical undertakings, and afterwards, down to the end of his life, in his astronomical labours.]

[Footnote 11: Holden, _op. cit._, p. 39.]

[Footnote 12: _Memoir of Caroline Herschel_, p. 37.]

[Footnote 13: See Holden's _Sir William Herschel_, p. 54.]

[Footnote 14: _An Original Theory or New Hypothesis of the Universe_, London, 1750. See also De Morgan's summary of his views in _Philosophical Magazine_, April, 1848.]

[Footnote 15: _Allgemeine Naturgeschichte und Theorie des Himmels_, 1755.]

[Footnote 16: _Cosmologische Briefe_, Augsburg, 1761.]

[Footnote 17: _The System of the World_, p. 125, London, 1800 (a translation of _Cosmologische Briefe_). Lambert regarded nebulae as composed of stars crowded together, but _not_ as external universes. In the case of the Orion nebula, indeed, he throws out such a conjecture, but afterwards suggests that it may form a centre for that one of the subordinate systems composing the Milky Way to which our sun belongs.]

[Footnote 18: _Opera Inedita_, t. i., p. 79.]

[Footnote 19: _Phil. Trans._, vol. lxxiii. (1783), p. 273. Pierre Prevost's similar investigation, communicated to the Berlin Academy of Sciences four months later, July 3, 1783, was inserted in the _Memoirs_ of that body for 1781, and thus _seems_ to claim a priority not its due.

Georg Simon Klugel at Halle gave about the same time an a.n.a.lytical demonstration of Herschel's result. Wolf, _Gesch. der Astronomie_, p.

733.]

[Footnote 20: _Phil. Trans._, vol. xcv., p. 233.]

[Footnote 21: _Ibid._, vol. xcvi., p. 205.]

[Footnote 22: "Ingens bolus devorandus est," Kepler admitted to Herwart in May, 1603.]

[Footnote 23: Described in "Praefatio Editoris" to _De Revolutionibus_, p. xix. (ed. 1854).]

[Footnote 24: _Opere_, t. i., p. 415.]

[Footnote 25: _Phil. Trans._, vol. xvii., p. 848.]

[Footnote 26: _Ibid._, vol. lxxii., p. 97.]

[Footnote 27: Doberck, _Observatory_, vol. ii., p. 110.]

[Footnote 28: _Phil. Trans._, vol. lvii., p. 249.]

[Footnote 29: _Ibid._, vol. lxxiv., p. 56.]

[Footnote 30: _Beobachtungen von Fixsterntrabanten_, 1778; and _De Novis in Coelo Sidereo Phaenomenis_, 1779.]

[Footnote 31: _Bibliographie_, p. 569.]

[Footnote 32: _Phil. Trans._, vol. lxxii., p. 162.]

[Footnote 33: _Ibid._, vol. lxxiii., p. 272.]

[Footnote 34: _Ibid._, vol. xciii., p. 340.]

[Footnote 35: _Phil. Trans._, vol. lxxv., p. 255.]

[Footnote 36: _Ibid._, vol. lxxix., pp. 214, 222.]

[Footnote 37: _Ibid._, vol. xcii., pp. 479, 495.]

[Footnote 38: _Phil. Trans._, vol. ci., p. 269.]

[Footnote 39: _Ibid._, vol. cvii., p. 311.]