A Popular History of Astronomy During the Nineteenth Century Part 14

Its construction was of the Newtonian kind, the observer looking into the side of the tube near its upper end, which a series of galleries and sliding stages enabled him to reach in any position. It has also, though rarely, been used without a second mirror, as a "Herschelian" reflector.

The splendour of the celestial objects as viewed with this vast "light-grasper" surpa.s.sed all expectation. "Never in my life," exclaimed Sir James South, "did I see such glorious sidereal pictures."[327] The orb of Jupiter produced an effect compared to that of the introduction of a coach-lamp into the telescope;[328] and certain star-cl.u.s.ters exhibited an appearance (we again quote Sir James South) "such as man before had never seen, and which for its magnificence baffles all description." But it was in the examination of the nebulae that the superiority of the new instrument was most strikingly displayed. A large number of these misty objects, which the utmost powers of Herschel's specula had failed to resolve into stars, yielded at once to the Parsonstown reflector; while many others showed under entirely changed forms through the disclosure of previously unseen details of structure.

One extremely curious result of the increase of light was the abolition of any sharp distinction between the two cla.s.ses of "annular" and "planetary" nebulae. Up to that time, only four ring-shaped systems--two in the northern and two in the southern hemisphere--were known to astronomers; they were now reinforced by five of the planetary kind, the discs of which were observed to be centrally perforated; while the definite margins visible in weaker instruments were replaced by ragged edges or filamentous fringes.

Still more striking was the discovery of an entirely new and most remarkable species of nebulae. These were termed "spiral," from the more or less regular convolutions, resembling the whorls of a sh.e.l.l, in which the matter composing them appeared to be distributed. The first and most conspicuous specimen of this cla.s.s was met with in April, 1845; it is situated in Canes Venatici, close to the tail of the Great Bear, and wore, in Sir J. Herschel's instruments, the aspect of a split ring encompa.s.sing a bright nucleus, thus presenting, as he supposed, a complete a.n.a.logue to the system of the Milky Way. In the Rosse mirror it shone out as a vast whirlpool of light--a stupendous witness to the presence of cosmical activities on the grandest scale, yet regulated by laws as to the nature of which we are profoundly ignorant. Professor Stephen Alexander of New Jersey, however, concluded, from an investigation (necessarily founded on highly precarious data) of the mechanical condition of these extraordinary agglomerations, that we see in them "the partially scattered fragments of enormous ma.s.ses once rotating in a state of dynamical equilibrium." He further suggested "that the separation of these fragments may still be in progress,"[329]

and traced back their origin to the disruption, through its own continually accelerated rotation, of a "primitive spheroid" of inconceivably vast dimensions. Such also, it was added (the curvilinear form of certain outliers of the Milky Way giving evidence of a spiral structure), is probably the history of our own cl.u.s.ter; the stars composing which, no longer held together in a delicately adjusted system like that of the sun and planets, are advancing through a period of seeming confusion towards an appointed goal of higher order and more perfect and harmonious adaptation.[330]

The cla.s.s of spiral nebulae included, in 1850, fourteen members, besides several in which the characteristic arrangement seemed partial or dubious.[331] A tendency in the exterior stars of other cl.u.s.ters to gather into curved branches (as in our Galaxy) was likewise noted; and the existence of unsuspected a.n.a.logies was proclaimed by the significant combination in the "Owl" nebula (a large planetary in Ursa Major)[332]

of the twisted forms of a spiral with the perforated effect distinctive of an annular nebula.

Once more, by the achievements of the Parsonstown reflector, the supposition of a "s.h.i.+ning fluid" filling vast regions of s.p.a.ce was brought into (as it has since proved) undeserved discredit. Although Lord Rosse himself rejected the inference, that because many nebulae had been resolved, all were resolvable, very few imitated his truly scientific caution; and the results of Bond's investigations[333] with the Harvard College refractor quickened and strengthened the current of prevalent opinion. It is now certain that the evidence furnished on both sides of the Atlantic as to the stellar composition of some conspicuous objects of this cla.s.s (notably the Orion and "Dumb-bell" nebulae) was delusive; but the spectroscope alone was capable of meeting it with a categorical denial. Meanwhile there seemed good ground for the persuasion, which now, for the last time, gained the upper hand, that nebulae are, without exception, true "island-universes," or a.s.semblages of distant suns.

Lord Rosse's telescope possesses a nominal power of 6,000--that is, it shows the moon as if viewed with the naked eye at a distance of forty miles. But this seeming advantage is neutralised by the weakening of the available light through excessive diffusion, as well as by the troubles of the surging sea of air through which the observation must necessarily be made. Professor Newcomb, in fact, doubts whether with _any_ telescope our satellite has ever been seen to such advantage as it would be if brought within 500 miles of the unarmed eye.[334]

The French opticians' rule of doubling the number of millimetres contained in the aperture of an instrument to find the highest magnifying power usually applicable to it, would give 3,600 as the maximum for the leviathan of Birr Castle; but in a climate like that of Ireland the occasions must be rare when even that limit can be reached.

Indeed, the experience acquired by its use plainly shows that atmospheric rather than mechanical difficulties impede a still further increase of telescopic power. Its construction may accordingly be said to mark the _ne plus ultra_ of effort in one direction, and the beginning of its conversion towards another. It became thenceforward more and more obvious that the conditions of observation must be ameliorated before any added efficacy could be given to it. The full effect of an uncertain climate in nullifying optical improvements was recognised, and the attention of astronomers began to be turned towards the advantages offered by more tranquil and more translucent skies.

Scarcely less important for the practical uses of astronomy than the optical qualities of the telescope is the manner of its mounting. The most admirable performance of the optician can render but unsatisfactory service if its mechanical accessories are ill-arranged or inconvenient.

Thus the astronomer is ultimately dependent upon the mechanician; and so excellently have his needs been served, that the history of the ingenious contrivances by which discoveries have been prepared would supply a subject (here barely glanced at) not far inferior in extent and instruction to the history of those discoveries themselves.

There are two chief modes of using the telescope, to which all others may be considered subordinate.[335] Either it may be invariably directed towards the south, with no motion save in the plane of the meridian, so as to intercept the heavenly bodies at the moment of transit across that plain; or it may be arranged so as to follow the daily revolution of the sky, thus keeping the object viewed permanently in sight instead of simply noting the instant of its flitting across the telescopic field.

The first plan is that of the "transit instrument," the second that of the "equatoreal." Both were, by a remarkable coincidence, introduced about 1690[336] by Olaus Romer, the brilliant Danish astronomer who first measured the velocity of light.

The uses of each are entirely different. With the transit, the really fundamental task of astronomy--the determination of the movements of the heavenly bodies--is mainly accomplished; while the investigation of their nature and peculiarities is best conducted with the equatoreal.

One is the instrument of mathematical, the other of descriptive astronomy. One furnishes the materials with which theories are constructed and the tests by which they are corrected; the other registers new facts, takes note of new appearances, sounds the depths and peers into every nook of the heavens.

The great improvement of giving to a telescope equatoreally mounted an automatic movement by connecting it with clockwork, was proposed in 1674 by Robert Hooke. Bradley in 1721 actually observed Mars with a telescope "moved by a machine that made it keep pace with the stars;"[337] and Von Zach relates[338] that he had once followed Sirius for twelve hours with a "heliostat" of Ramsden's construction. But these eighteenth-century attempts were of no practical effect. Movement by clockwork was virtually a complete novelty when it was adopted by Fraunhofer in 1824 to the Dorpat refractor. By simply giving to an axis unvaryingly directed towards the celestial pole an equable rotation with a period of twenty-four hours, a telescope attached to it, and pointed in _any_ direction, will trace out on the sky a parallel of declination, thus necessarily accompanying the movement of any star upon which it may be fixed. It accordingly forms part of the large sum of Fraunhofer's merits to have secured this inestimable advantage to observers.

Sir John Herschel considered that La.s.sell's application of equatoreal mounting to a nine-inch Newtonian in 1840 made an epoch in the history of "that eminently British instrument, the reflecting telescope."[339]

Nearly a century earlier,[340] it is true, Short had fitted one of his Gregorians to a complicated system of circles in such a manner that, by moving a handle, it could be made to follow the revolution of the sky; but the arrangement did not obtain, nor did it deserve, general adoption. La.s.sell's plan was a totally different one; he employed the crossed axes of the true equatoreal, and his success removed, to a great extent, the fatal objection of inconvenience in use, until then unanswerably urged against reflectors. The very largest of these can now be mounted equatoreally; even the Rosse, within its limited range, has been for some years provided with a movement by clockwork along declination-parallels.

The art of accurately dividing circular arcs into the minute equal parts which serve as the units of astronomical measurement, remained, during the whole of the eighteenth century, almost exclusively in English hands. It was brought to a high degree of perfection by Graham, Bird and Ramsden, all of whom, however, gave the preference to the old-fas.h.i.+oned mural quadrant and zenith-sector over the entire circle, which Romer had already found the advantage of employing. The five-foot vertical circle, which Piazzi with some difficulty induced Ramsden to complete for him in 1789, was the first divided instrument constructed in what may be called the modern style. It was provided with magnifiers for reading off the divisions (one of the neglected improvements of Romer), and was set up above a smaller horizontal circle, forming an "alt.i.tude and azimuth"

combination (again Romer's invention), by which both the elevation of a celestial object above the horizon and its position as referred to the horizon could be measured. In the same year, Borda invented the "repeating circle" (the principle of which had been suggested by Tobias Mayer in 1756[341]), a device for exterminating, so far as possible, errors of graduation by _repeating_ an observation with different parts of the limb. This was perhaps the earliest systematic effort to correct the imperfections of instruments by the manner of their use.

The manufacture of astronomical circles was brought to a very refined state of excellence early in the nineteenth century by Reichenbach at Munich, and after 1818 by Repsold at Hamburg. Bessel states[342] that the "reading-off" on an instrument of the kind by the latter artist was accurate to about 1/80th of a human hair. Meanwhile the traditional reputation of the English school was fully sustained; and Sir George Airy did not hesitate to express his opinion that the new method of graduating circles, published by Troughton in 1809,[343] was the "greatest improvement ever made in the art of instrument-making."[344]

But a more secure road to improvement than that of mere mechanical exactness was pointed out by Bessel. His introduction of a regular theory of instrumental errors might almost be said to have created a new art of observation. Every instrument, he declared in memorable words,[345] must be twice made--once by the artist, and again by the observer. Knowledge is power. Defects that are ascertained and can be allowed for are as good as non-existent. Thus the truism that the best instrument is worthless in the hands of a careless or clumsy observer, became supplemented by the converse maxim, that defective appliances may, through skilful use, be made to yield valuable results. The Konigsberg observations--of which the first instalment was published in 1815--set the example of regular "reduction" for instrumental errors.

Since then, it has become an elementary part of an astronomer's duty to study the _idiosyncrasy_ of each one of the mechanical contrivances at his disposal, in order that its inevitable, but now certified deviations from ideal accuracy may be included amongst the numerous corrections by which the pure essence of even approximate truth is distilled from the rude impressions of sense.

Nor is this enough; for the casual circ.u.mstances attending each observation have to be taken into account with no less care than the inherent or _const.i.tutional_ peculiarities of the instrument with which it is made. There is no "once for all" in astronomy. Vigilance can never sleep; patience can never tire. Variable as well as constant sources of error must be anxiously heeded; one infinitesimal inaccuracy must be weighed against another; all the forces and vicissitudes of nature--frosts, dews, winds, the interchanges of heat, the disturbing effects of gravity, the s.h.i.+verings of the air, the tremors of the earth, the weight and vital warmth of the observer's own body, nay, the rate at which his brain receives and transmits its impressions, must all enter into his calculations, and be sifted out from his results.

It was in 1823 that Bessel drew attention to discrepancies in the times of transits given by different astronomers.[346] The quant.i.ties involved were far from insignificant. He was himself nearly a second in advance of all his contemporaries, Argelander lagging behind him as much as a second and a quarter. Each individual, in fact, was found to have a certain definite _rate of perception_, which, under the name of "personal equation," now forms so important an element in the correction of observations that a special instrument for accurately determining its amount in each case is in actual use at Greenwich.

Such are the refinements upon which modern astronomy depends for its progress. It is a science of hairbreadths and fractions of a second. It exists only by the rigid enforcement of arduous accuracy and unwearying diligence. Whatever secrets the universe still has in store for man will only be communicated on these terms. They are, it must be acknowledged, difficult to comply with. They involve an unceasing struggle against the infirmities of his nature and the instabilities of his position. But the end is not unworthy the sacrifices demanded. One additional ray of light thrown on the marvels of creation--a single, minutest encroachment upon the strongholds of ignorance--is recompense enough for a lifetime of toil. Or rather, the toil is its own reward, if pursued in the lofty spirit which alone becomes it. For it leads through the abysses of s.p.a.ce and the unending vistas of time to the very threshold of that infinity and eternity of which the disclosure is reserved for a life to come.

FOOTNOTES:

[Footnote 305: Grant, _Hist. Astr._, p. 527.]

[Footnote 306: _Optica Promota_, p. 93.]

[Footnote 307: _Phil. Trans._, vol. x.x.xii., p. 383.]

[Footnote 308: _Ibid._, vol. xc., p. 65.]

[Footnote 309: Ca.s.segrain, a Frenchman, subst.i.tuted in 1672 a _convex_ for a _concave_ secondary speculum. The tube was thereby enabled to be shortened by twice the focal length of the mirror in question. The great Melbourne reflector (four feet aperture, by Grubb) is constructed upon this plan.]

[Footnote 310: _Phil. Trans._, vol. civ., p. 275, _note_.]

[Footnote 311: _Phil. Trans._, vol. xc., p. 70. With the forty-foot, however, only very moderate powers seemed to have been employed, whence Dr. Robinson argued a deficiency of defining power. _Proc. Roy. Irish Ac._, vol. ii., p. 11.]

[Footnote 312: _Phil. Trans._, vol. lxxi., p. 492.]

[Footnote 313: It is remarkable that, as early as 1695, the possibility of an achromatic combination was inferred by David Gregory from the structure of the human eye. See his _Catoptricae et Dioptricae Sphericae Elementa_, p. 98.]

[Footnote 314: Wolf, _Biographien_, Bd. ii., p. 301.]

[Footnote 315: _Month. Not._, vol. i., p. 153. _note_.]

[Footnote 316: Henrivaux, _Encyclopedie Chimique_, t. v., fasc. 5, p.

363.]

[Footnote 317: See _ante_, p. 83.]

[Footnote 318: _Phil. Trans._, vol. vii., p. 4007.]

[Footnote 319: J. Herschel, _The Telescope_, p. 39.]

[Footnote 320: _Month. Not._, vol. xxix., p. 125.]

[Footnote 321: A slight excess of copper renders the metal easier to work, but liable to tarnish. Robinson, _Proc. Roy. Irish Ac._, vol. ii., p. 4.]

[Footnote 322: _Brit. a.s.s._, 1843, Dr. Robinson's closing Address.

_Athenaeum_, Sept. 23, p. 866.]

[Footnote 323: _The Telescope_, p. 82.]

[Footnote 324: Lord Rosse in _Phil. Trans._, vol. cxl., p. 302.]

[Footnote 325: This method is the same in principle with that applied by Grubb in 1834 to a 15-inch speculum for the observatory of Armagh.

_Phil. Trans._, vol. clix., p. 145.]

[Footnote 326: Robinson, _Proc. Roy. Ir. Ac._, vol. iii., p. 120.]

[Footnote 327: _Astr. Nach._, No. 536.]

[Footnote 328: Airy, _Month. Not._, vol. ix., p. 120.]