Sir William Herschel: His Life and Works Part 13

It is still in his capacity of an observer--an acute and wise one--that HERSCHEL is considered. But this was the least of his gifts. This vast ma.s.s of material was not left in this state: it served him for a stepping-stone to larger views of the nature and extent of the nebulous matter itself.

His views on the nature of nebulae underwent successive changes. At first he supposed all nebulae to be but aggregations of stars. The logic was simple. To the naked eye there are many groups of stars which appear nebulous. _Praesepe_ is, perhaps, the best example. The slightest telescopic power applied to such groups alters the nebulous appearance, and shows that it comes from the combined and confused light of discrete stars. Other groups which remain nebulous in a seven-foot telescope, become stellar in a ten-foot. The nebulosity of the ten-foot can be resolved into stars by the twenty-foot, and so on. The nebulae which remained still unresolved, it was reasonable to conclude, would yield to higher power, and generally a nebula was but a group of stars removed to a great distance. An increase of telescopic power was alone necessary to demonstrate this.[37]

"Nebulae can be selected so that an insensible gradation shall take place from a coa.r.s.e cl.u.s.ter like the _Pleiades_ down to a milky nebulosity like that in _Orion_, every intermediate step being represented. This tends to confirm the hypothesis that all are composed of stars more or less remote."

So, at first, HERSCHEL believed that his twenty-foot telescope was of power sufficient to fathom the Milky Way, that is, to see through it and beyond it, and to reduce all its nebulosities to true groups of stars.

In 1791 he published a memoir on _Nebulous Stars_, in which his views were completely changed. He had found a nebulous star, the sixty-ninth of his Cla.s.s IV., to which his reasons would not apply. In the centre of it was a bright star; around the star was a halo gradually diminishing in brightness from the star outward, and perfectly circular. It was clear the two parts, star and nebula, were connected, and thus at the same distance from us.

There were two possible solutions only. Either the whole ma.s.s was, _first_, composed of stars, in which case the nucleus would be enormously larger than the other stars of its stellar magnitude elsewhere in the sky, or the stars which made up the halo indefinitely small; or, _second_, the central nucleus was indeed a star, but a star surrounded with "a shining fluid, of a nature totally unknown to us."

The long strata of nebulae, which he had before described under the name of "telescopic Milky Ways," might well be accounted for by ma.s.ses of this fluid lying beyond the regions of the seventh-magnitude stars. This fluid might exist independently of stars. If it is self-luminous, it seems more fit to produce a star by its condensation, than to depend upon the star for its own existence. Such were a few of the theorems to which his discovery of this nebula led him. The hypothesis of an elastic _shining fluid_ existing in s.p.a.ce, sometimes in connection with stars, sometimes distinct from them, was adopted and never abandoned. How well the spectroscope has confirmed this idea it is not necessary to say.

We know the shining fluid does exist, and in late years we have seen the reverse of the process imagined by HERSCHEL. A star has actually, under our eyes, become a planetary nebula, and the cycle of which he gave the first terms is complete.

In five separate memoirs (1802, 1811, 1814, 1817, and 1818) HERSCHEL elaborated his views of the sidereal system. The whole extent of his views must be gained from the extended memoirs themselves. Here only the merest outline can be given.

In 1802 there is a marshaling of the various objects beyond our solar system. The stars themselves may be _insulated_, or may belong to _binary_ or _multiple_ systems, to _cl.u.s.ters_ and groups, or to grand groups like the Milky Way. Nebulae may have any of the forms which have been described; and, in 1811, he gives examples of immense s.p.a.ces in the sky covered with diffused and very faint nebulosity. "Its abundance exceeds all imagination."[38] These ma.s.ses of nebular matter are the seats of attracting forces, and these forces must produce condensation.

When a nebula has more than one preponderating seat of attracting matter, it may in time be divided, and the double nebulae have had such an origin. When nebulae appear to us as round ma.s.ses, they are in reality globular in form, and this form is at once the effect and the proof of a gravitating cause.

The central brightness of nebulae points out the seat of the attraction; and the completeness of the approximation to a spherical form points out the length of time that the gravitating forces have been at work. Those nebulae (and cl.u.s.ters) which are most perfect in the globular form, have been longest exposed to central forces. The planetary nebulae are the oldest in our system. They must have a rotatory motion on their axes.

By progressive condensation planetary nebulae may be successively converted into bright stellar nebulae, or into nebulous stars, and these again, by the effects of the same cause, into insulated or double stars.

This chain of theorems, laid down in the memoir of 1811, is enforced in 1814 with examples which show how the nebulous appearance may grow into the sidereal. HERSCHEL selects from the hundreds of instances in his note-books, nebulae in every stage of progress, and traces the effect of condensation and of cl.u.s.tering power through all its course, even to the final breaking up of the Milky Way itself.

The memoirs of 1817 and 1818 add little to the general view of the physical const.i.tution of the heavens. They are attempts to gain a scale of celestial measures by which we may judge of the distances of the stars and cl.u.s.ters in which these changes are going on.

There is little to change in HERSCHEL'S statement of the general construction of the heavens. It is the groundwork upon which we have still to build. Every astronomical discovery and every physical fact well observed is material for the elaboration of its details or for the correction of some of its minor points. As a scientific conception it is perhaps the grandest that has ever entered into the human mind. As a study of the height to which the efforts of one man may go, it is almost without a parallel. The philosopher who will add to it to-day, will have his facts and his methods ready to his hands. HERSCHEL presents the almost unique example of an eager observer marshaling the mult.i.tude of single instances, which he himself has laboriously gathered, into a compact and philosophic whole. In spite of minor errors and defects, his ideas of the nature of the sidereal universe have prevailed, and are to-day the unacknowledged basis of our every thought upon it. Some of its most secret processes have been worked out by him, and the paths which he pointed out are those along which our advances must be made.

In concluding this condensed account of HERSCHEL'S scientific labors, it behoves us to remember that there was nothing due to accident in his long life. He was born with the faculties which fitted him for the gigantic labors which he undertook, and he had the firm basis of energy and principle which kept him steadily to his work.

As a practical astronomer he remains without an equal. In profound philosophy he has few superiors. By a kindly chance he can be claimed as the citizen of no one country. In very truth his is one of the few names which belong to the whole world.

FOOTNOTES:

[31] JAMES SHORT, F.R.S. (1710-1768), and JOHN DOLLOND, F.R.S.

(1706-1761), were the most celebrated makers of telescopes of their day.

The six-foot Newtonian reflectors of SHORT (aperture 9.4 inches), and the forty-six-inch achromatics of DOLLOND (aperture 3.6 inches), were highly esteemed. The Royal Observatory of Greenwich possessed, in 1765, one of each cla.s.s. In a comparative trial of SHORT'S telescope, at Greenwich, and one of HERSCHEL'S first telescopes, the latter was adjudged greatly superior.

[32] At least _one_ of these telescopes had the princ.i.p.al mirror made of gla.s.s instead of metal.--_Philosophical Transactions_, 1803.

[33] The following extract from FOURIER'S _eloge_ of HERSCHEL is of interest in this connection. The sum first appropriated by the king was 2,000. This was afterwards raised to 4,000, and a sum of 200 yearly was given for maintenance.

"L'histoire doit conserver a jamais la reponse de ce prince a un etranger celebre [LALANDE?] qui le remerciait des sommes considerables accordees pour les progres de l'astronomie. 'Je fais les depenses de la guerre,' dit le roi, 'parcequ'elles sont necessaires; quant a celles des sciences, il m'est agreable des les ordonner; leur objet ne coute point des larmes, et honore l'humanite.'"

LALANDE'S own account is a little different. He says the king exclaimed: "Ne vaut-il pas mieux employer son argent a cela qu'a faire tuer des hommes?"

[34] The memoirs on the parallaxes of stars, written by various astronomers from 1750 to 1800, were mainly directed to the improvement of the methods, or to the discovery of the parallax of some particular star. For example, LACAILLE'S observations of _Sirius_, at the Cape of Good Hope, had resulted in a parallax of 9" for that star--a quant.i.ty over forty times too large.

[35] HERSCHEL accepted, as did all his cotemporaries, the Newtonian or corpuscular theory of light.

[36] Thus the position of small stars critically situated in the centre, or on the edges of the nebulae was always noted. Many of the descriptions are given in the published papers, but the publication of the diagrams would be an immense help to this branch of astronomy.

D'ARREST in his reduction of HERSCHEL'S nebula observations (1856) writes: "Gewiss ware es vom hochsten Interesse fur die Entwickelung, welche hoffentlich auch dieser Zweig der beobachtenden Astronomie zukunftig erhalten wird, wenn die HERSCHEL'Schen Beobachtungen in der Ausfuhrlichkeit in welcher sie, verschiedenen Andeutungen zufolge, _handschriftlich_ vorhanden sind, veroffentlicht wurden. Es schliesst sich dieser Wunsch in Betreff der Nebelflecken lebhaft an den an, welcher, schon vor einem Jahrzehnt nach Veroffentlichung der 400 noch unedirten _star-gauges_ von gewichtigerer Seite her geaussert wurde."

In this all must agree who have a knowledge of the direction in which we must look for advances in the difficult and important questions of the distance, the motions, and the changes of the nebulae. Almost the only aid to be looked for from the older observations must come from such diagrams, and we may safely say that the publication of this priceless material, just as it stands, would carry our exact data back from 1833 to 1786, or no less than forty-seven years.

[37] Long after HERSCHEL had abandoned this idea, it continued current among astronomers. The successes of Lord ROSSE'S telescope perpetuated to the middle of the nineteenth century an erroneous view which HERSCHEL had given up in 1791.

[38] These have never been re-observed. They should be sought for with a powerful refractor, taking special precautions against the illumination of the field of view from neighboring bright stars.

HERSCHEL'S reflectors were specially open to illusions produced in this way. His observations probably will remain untested until some large telescope is used in the way he adopted, _i. e._, in sweeping.

BIBLIOGRAPHY.

I.--LIST OF THE PUBLISHED WRITINGS OF WILLIAM HERSCHEL ON ASTRONOMICAL SUBJECTS.

[In chronological order.]

_N.B.--In general, translations and abstracts of those which appeared in periodicals are not noticed here. I have made exceptions in the more important cases._

[Solution of a prize question. _See_ this book, page 46.]

_Ladies' Diary_, 1779.

Astronomical observations on the periodical star in _Collo Ceti_.

_Phil. Trans._, 1780, p. 338.

Astronomical observations relating to the mountains of the moon.

_Phil. Trans._, 1780, p. 507.

Astronomical observations on the rotation of the planets round their axes, made with a view to determine whether the earth's diurnal motion is perfectly equable.

_Phil. Trans._, 1781, p. 115.

Account of a comet. [Dated 13th March, 1781. This was _Ura.n.u.s_.]

_Phil. Trans._, 1781, p. 492.

On the parallax of the fixed stars.

_Phil. Trans._, 1782, p. 82.

Catalogue of double stars.

_Phil. Trans._, 1782, p. 112: translation in _Bode's Jahrbuch_, 1786, p. 187.