Part 13 (1/2)

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 mass of material was not left in this state: it served hier 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 siroups of stars which appear nebulous _Praesepe_ is, perhaps, the best exaroups alters the nebulous appearance, and shows that it coht 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 rereat distance An increase of telescopic poas alone necessary to demonstrate this[37]

”Nebulae can be selected so that an insensible gradation shall take place from a coarse cluster like the _Pleiades_ down to a milky nebulosity like that in _Orion_, every inter 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 fathoh 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 vieere coed He had found a nebulous star, the sixty-ninth of his Class IV, to which his reasons would not apply In the centre of it was a bright star; around the star was a halo gradually dihtness 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 mass was, _first_, composed of stars, in which case the nucleus would be enornitude 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 shi+ning fluid, of a nature totally unknown to us”

The long strata of nebulae, which he had before described under the naht well be accounted for by ions of the seventh-ht 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 hi in space, sometimes in connection with stars, sometimes distinct from them, was adopted and never abandoned Hoell the spectroscope has confirmed this idea it is not necessary to say

We know the shi+ning fluid does exist, and in late years we have seen the reverse of the process iined by HERSCHEL A star has actually, under our eyes, becoave the first terms is complete

In five separate memoirs (1802, 1811, 1814, 1817, and 1818) HERSCHEL elaborated his views of the sidereal systeained from the extended iven

In 1802 there is aof the various objects beyond our solar syste to _binary_ or _roups like the Milky Way Nebulae may have any of the forives examples of immense spaces in the sky covered with diffused and very faint nebulosity ”Its abundance exceeds all iination”[38] Theseforces, and these forces must produce condensation

When a nebula hasmatter, it may in tiin When nebulae appear to us as round lobular in forravitating cause

The central brightness of nebulae points out the seat of the attraction; and the completeness of the approxith of ti forces have been at work Those nebulae (and clusters) which are est exposed to central forces The planetary nebulae are the oldest in our system They ressive condensation planetary nebulae ht 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 exarow 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 clustering power through all its course, even to the final breaking up of the Milky Way itself

The eneral view of the physical constitution of the heavens They are atteain a scale of celestial e of the distances of the stars and clusters in which these changes are going on

There is little to change in HERSCHEL'S stateroundwork 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 randest that has ever entered into the huht to which the efforts of one o, it is almost without a parallel The philosopher ill add to it to-day, will have his facts and his methods ready to his hands HERSCHEL presents the al the le instances, which he hiathered, 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 hi which our advancesthis condensed account of HERSCHEL'S scientific labors, it behoves us to re life He was born with the faculties which fitted hiantic labors which he undertook, and he had the firy 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 na to the whole world

FOOTNOTES:

[31] JAMES SHORT, FRS (1710-1768), and JOHN DOLLOND, FRS

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

The six-foot Newtonian reflectors of SHORT (aperture 94 inches), and the forty-six-inch achrohly esteemed The Royal Observatory of Greenwich possessed, in 1765, one of each class 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 principal lass instead of metal--_Philosophical Transactions_, 1803

[33] The following extract froe_ of HERSCHEL is of interest in this connection The su was 2,000 This was afterwards raised to 4,000, and a suiven for maintenance

”L'histoire doit conserver a jaer celebre [LALANDE?] qui le reres de l'astronouerre,' dit le roi, 'parcequ'elles sont necessaires; quant a celles des sciences, il reable 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 exclaient 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 quantity over forty tie