[Footnote 288: _Annuaire_, 1836, p. 233.]

[Footnote 289: _Cosmos_, vol. i., p. 90, _note_ (Otte's trans.).]

[Footnote 290: Herschel, _Outlines of Astronomy_, p. 399, 9th ed.]

[Footnote 291: _Outlines_, p. 398.]

[Footnote 292: Boguslawski calculated that it extended on the 21st of March to 581 millions.--_Report. Brit. a.s.s._, 1845, p. 89.]

[Footnote 293: _Comptes Rendus_, t. xvi., p. 919.]

[Footnote 294: _Observatory_, vol. xxiv., p. 167; Astr. Nach., No.

3,320.]

[Footnote 295: Piazzi noticed a considerable increase of l.u.s.tre in a very faint star of the twelfth magnitude viewed through a comet. Madler, _Reden_, etc., p. 248, _note_.]

[Footnote 296: _Astr. Jahrbuch_, 1828, p. 151.]

[Footnote 297: Madler, _Gesch. d. Astr._, Bd. ii., p. 412.]

[Footnote 298: _Recueil de l'Ac. Imp. de St. Petersbourg_, 1835, p.

143.]

[Footnote 299: Guillemin's _World of Comets_, trans, by J. Glaisher, p.

294, _note_.]

[Footnote 300: _Month. Not._, vol. viii., p. 9.]

[Footnote 301: A real, though only partial stoppage of light seems indicated by Herschel's observations on the comet of 1807. Stars seen through the tail, October 18, lost much of their l.u.s.tre. One near the head was only faintly visible by glimpses. _Phil. Trans._, vol. xcvii., p. 153.]

[Footnote 302: Arago, _Annuaire_, 1832, p. 205.]

[Footnote 303: _Ibid._, 1891, p. 290.]

[Footnote 304: Viz., Encke's, Biela's, Faye's, and Brorsen's.]

CHAPTER VI

_INSTRUMENTAL ADVANCES_

It is impossible to follow with intelligent interest the course of astronomical discovery without feeling some curiosity as to the means by which such surpa.s.sing results have been secured. Indeed, the bare acquaintance with _what_ has been achieved, without any corresponding knowledge of _how_ it has been achieved, supplies food for barren wonder rather than for fruitful and profitable thought. Ideas advance most readily along the solid ground of practical reality, and often find true sublimity while laying aside empty marvels. Progress is the result, not so much of sudden flights of genius, as of sustained, patient, often commonplace endeavour; and the true lesson of scientific history lies in the close connection which it discloses between the most brilliant developments of knowledge and the faithful accomplishment of his daily task by each individual thinker and worker.

It would be easy to fill a volume with the detailed account of the long succession of optical and mechanical improvements by means of which the observation of the heavens has been brought to its present degree of perfection; but we must here content ourselves with a summary sketch of the chief amongst them. The first place in our consideration is naturally claimed by the telescope.

This marvellous instrument, we need hardly remind our readers, is of two distinct kinds--that in which light is gathered together into a focus by _refraction_, and that in which the same end is attained by _reflection_. The image formed is in each case viewed through a magnifying lens, or combination of lenses, called the eye-piece. Not for above a century after the "optic gla.s.ses" invented or stumbled upon by the spectacle-maker of Middelburg (1608) had become diffused over Europe, did the reflecting telescope come, even in England, the place of its birth, into general use. Its principle (a sufficiently obvious one) had indeed been suggested by Mersenne as early as 1639;[305] James Gregory in 1663[306] described in detail a mode of embodying that principle in a practical shape; and Newton, adopting an original system of construction, actually produced in 1668 a tiny speculum, one inch across, by means of which the apparent distance of objects was reduced thirty-nine times. Nevertheless, the exorbitantly long tubeless refractors, introduced by Huygens, maintained their reputation until Hadley exhibited to the Royal Society, January 12, 1721,[307] a reflector of six inches aperture, and sixty-two in focal length, which rivalled in performance, and of course indefinitely surpa.s.sed in manageability, one of the "aerial" kind of 123 feet.

The concave-mirror system now gained a decided ascendant, and was brought to unexampled perfection by James Short of Edinburgh during the years 1732-68. Its resources were, however, first fully developed by William Herschel. The energy and inventiveness of this extraordinary man marked an epoch wherever they were applied. His ardent desire to measure and gauge the stupendous array of worlds which his specula revealed to him, made him continually intent upon adding to their "s.p.a.ce-penetrating power" by increasing their light-gathering surface. These, as he was the first to explain,[308] are in a constant proportion one to the other.

For a telescope with twice the linear aperture of another will collect four times as much light, and will consequently disclose an object four times as faint as could be seen with the first, or, what comes to the same, an object equally bright at twice the distance. In other words, it will possess double the s.p.a.ce-penetrating power of the smaller instrument. Herschel's great mirrors--the first examples of the giant telescopes of modern times--were then primarily engines for extending the bounds of the visible universe; and from the sublimity of this "final cause" was derived the vivid enthusiasm which animated his efforts to success.

It seems probable that the seven-foot telescope constructed by him in 1775--that is within little more than a year after his experiments in shaping and polishing metal had begun--already exceeded in effective power any work by an earlier optician; and both his skill and his ambition rapidly developed. His efforts culminated, after mirrors of ten, twenty, and thirty feet focal length had successively left his hands, in the gigantic forty-foot, completed August 28, 1789. It was the first reflector in which only a single mirror was employed. In the "Gregorian" form, the focussed rays are, by a second reflection from a small concave[309] mirror, thrown _straight back_ through a central aperture in the larger one, behind which the eye-piece is fixed. The object under examination is thus seen in the natural direction. The "Newtonian," on the other hand, shows the object in a line of sight at right angles to the true one, the light collected by the speculum being diverted to one side of the tube by the interposition of a small plane mirror, situated at an angle of 45 to the axis of the instrument. Upon these two systems Herschel worked until 1787, when, becoming convinced of the supreme importance of economising light (necessarily wasted by the second reflection), he laid aside the small mirror of his forty-foot then in course of construction, and turned it into a "front-view"

reflector. This was done--according to the plan proposed by Lemaire in 1732--by slightly inclining the speculum so as to enable the image formed by it to be viewed with an eye-gla.s.s fixed at the upper margin of the tube. The observer thus stood with his back turned to the object he was engaged in scrutinising.

The advantages of the increased brilliancy afforded by this modification were strikingly ill.u.s.trated by the discovery, August 28 and September 17, 1789, of the two Saturnian satellites nearest the ring.

Nevertheless, the monster telescope of Slough cannot be said to have realised the sanguine expectations of its constructor. The occasions on which it could be usefully employed were found to be extremely rare. It was injuriously affected by every change of temperature. The great weight (25 cwt.) of a speculum four feet in diameter rendered it peculiarly liable to distortion. With all imaginable care, the delicate l.u.s.tre of its surface could not be preserved longer than two years,[310]

when the difficult process of repolishing had to be undertaken. It was accordingly never used after 1811, when, having _gone blind_ from damp, it lapsed by degrees into the condition of a museum inmate.

The exceedingly high magnifying powers employed by Herschel const.i.tuted a novelty in optical astronomy, to which he attached great importance.

The work of ordinary observation would, however, be hindered rather than helped by them. The attempt to increase in this manner the efficacy of the telescope is speedily checked by atmospheric, to say nothing of other difficulties. Precisely in the same proportion as an object is magnified, the disturbances of the medium through which it is seen are magnified also. Even on the clearest and most tranquil nights, the air is never for a moment really still. The rays of light traversing it are continually broken by minute fluctuations of refractive power caused by changes of temperature and pressure, and the currents which these engender. With such luminous quiverings and waverings the astronomer has always more or less to reckon; their absence is simply a question of degree; if sufficiently magnified, they are at all times capable of rendering observation impossible.

Thus, such powers as 3,000, 4,000, 5,000, even 6,652,[311] which Herschel now and again applied to his great telescopes, must, save on the rarest occasions, prove an impediment rather than an aid to vision.

They were, however, used by him only for special purposes, experimentally, not systematically, and with the clearest discrimination of their advantages and drawbacks. It is obvious that perfectly different ends are subserved by increasing the _aperture_ and by increasing the _power_ of a telescope. In the one case, a larger quant.i.ty of light is captured and concentrated; in the other, the same amount is distributed over a wider area. A diminution of brilliancy in the image accordingly attends, _coeteris paribus_, upon each augmentation of its apparent size. For this reason, such faint objects as nebulae are most successfully observed with moderate powers applied to instruments of a great capacity for light, the details of their structure actually disappearing when highly magnified. With stellar groups the reverse is the case. Stars cannot be magnified, simply because they are too remote to have any sensible dimensions; but the s.p.a.ce between them can. It was thus for the purpose of dividing very close double stars that Herschel increased to such an unprecedented extent the magnifying capabilities of his instruments; and to this improvement incidentally the discovery of Ura.n.u.s, March 13, 1781,[312]

was due. For by the examination with strong lenses of an object which, even with a power of 227, presented a suspicious appearance, he was able at once to p.r.o.nounce its disc to be real, not merely "spurious," and so to distinguish it unerringly from the crowd of stars amidst which it was moving.

While the reflecting telescope was astonishing the world by its rapid development in the hands of Herschel, its unpretending rival was slowly making its way towards the position which the future had in store for it. The great obstacle which long stood in the way of the improvement of refractors was the defect known as "chromatic aberration." This is due to no other cause than that which produces the rainbow and the spectrum--the separation, or "dispersion" in their pa.s.sage through a refracting medium, of the variously coloured rays composing a beam of white light. In an ordinary lens there is no common point of concentration; each colour has its own separate focus; and the resulting image, formed by the superposition of as many images as there are hues in the spectrum, is indefinitely terminated with a tinted border, eminently baffling to exactness of observation.

The extravagantly long telescopes of the seventeenth century were designed to _avoid_ this evil (as well as another source of indistinct vision in the spherical shape of lenses); but no attempt to _remedy_ it was made until an Ess.e.x gentleman succeeded, in 1733, in so combining lenses of flint and crown gla.s.s as to produce refraction without colour.[313] Mr. Chester More Hall was, however, equally indifferent to fame and profit, and took no pains to make his invention public. The _effective_ discovery of the achromatic telescope was, accordingly, reserved for John Dollond, whose method of correcting at the same time chromatic and spherical aberration was laid before the Royal Society in 1758. Modern astronomy may be said to have been thereby rendered possible. Refractors have always been found better suited than reflectors to the ordinary work of observatories. They are, so to speak, of a more robust, as well as of a more plastic nature. They suffer less from vicissitudes of temperature and climate. They retain their efficiency with fewer precautions and under more trying circ.u.mstances.

Above all, they co-operate more readily with mechanical appliances, and lend themselves with far greater facility to purposes of exact measurement.

A practical difficulty, however, impeded the realisation of the brilliant prospects held out by Dollond's invention. It was found impossible to procure flint-gla.s.s, such as was needed for optical use--that is, of perfectly h.o.m.ogeneous quality--except in fragments of insignificant size. Discs of more than two or three inches in diameter were of extreme rarity; and the crushing excise duty imposed upon the article by the financial unwisdom of the Government, both limited its production, and, by rendering experiments too costly for repet.i.tion, barred its improvement.

Up to this time, Great Britain had left foreign compet.i.tors far behind in the instrumental department of astronomy. The quadrants and circles of Bird, Cary and Ramsden were unapproached abroad. The reflecting telescope came into existence and reached maturity on British soil. The refracting telescope was cured of its inherent vices by British ingenuity. But with the opening of the nineteenth century, the almost unbroken monopoly of skill and contrivance which our countrymen had succeeded in establishing was invaded, and British workmen had to be content to exchange a position of supremacy for one of at least partial temporary inferiority.

Somewhat about the time that Herschel set about polishing his first speculum, Pierre Louis Guinand, a Swiss artisan, living near Chaux-de-Fonds, in the canton of Neuchatel, began to grind spectacles for his own use, and was thence led on to the rude construction of telescopes by fixing lenses in pasteboard tubes. The sight of an England achromatic stirred a higher ambition, and he took the first opportunity of procuring some flint gla.s.s from England (then the only source of supply), with the design of imitating an instrument the full capabilities of which he was destined to be the humble means of developing. The English gla.s.s proving of inferior quality, he conceived the possibility, unaided and ignorant of the art as he was, of himself making better, and spent seven years (1784-90) in fruitless experiments directed to that end. Failure only stimulated him to enlarge their scale. He bought some land near Les Brenets, constructed upon it a furnace capable of melting two quintals of gla.s.s, and reducing himself and his family to the barest necessaries of life, he poured his earnings (he at this time made bells for repeaters) unstintingly into his crucibles.[314] His undaunted resolution triumphed. In 1799 he carried to Paris and there showed to Lalande several discs of flawless crystal four to six inches in diameter. Lalande advised him to keep his secret, but in 1805 he was induced to remove to Munich, where he became the instructor of the immortal Fraunhofer. His return to Les Brenets in 1814 was signalised by the discovery of an ingenious mode of removing striated portions of gla.s.s by breaking and re-soldering the product of each melting, and he eventually attained to the manufacture of perfect discs up to 18 inches in diameter. An object-gla.s.s for which he had furnished the material to Cauchoix, procured him, in 1823, a royal invitation to settle in Paris; but he was no longer equal to the change, and died at the scene of his labours, February 13 following.

This same lens (12 inches across) was afterwards purchased by Sir James South, and the first observation made with it, February 13, 1830, disclosed to Sir John Herschel the sixth minute star in the central group of the Orion nebula, known as the "trapezium."[315] Bequeathed by South to Trinity College, Dublin, it was employed at the Dunsink Observatory by Brunnow and Ball in their investigations of stellar parallax. A still larger objective (of nearly 14 inches) made of Guinand's gla.s.s was secured in Paris, about the same time, by Mr. Edward Cooper of Markree Castle, Ireland. The peculiarity of the method discovered at Les Brenets resided in the manipulation, not in the quality of the ingredients; the secret, that is to say, was not chemical, but mechanical.[316] It was communicated by Henry Guinand (a son of the inventor) to Bontemps, one of the directors of the gla.s.sworks at Choisy-le-Roi, and by him transmitted to Messrs. Chance of Birmingham, with whom he entered into partnership when the revolutionary troubles of 1848 obliged him to quit his native country. The celebrated American opticians, Alvan Clark & Sons, derived from the Birmingham firm the materials for some of their early telescopes, notably the 19-inch Chicago and 26-inch Washington equatoreals; but the discs for the great Lick refractor, and others shaped by them in recent years, have been supplied by Feil of Paris.

Two distinguished amateurs, meanwhile, were preparing to rea.s.sert on behalf of reflecting instruments their claim to the place of honour in the van of astronomical discovery. Of Mr. La.s.sell's specula something has already been said.[317] They were composed of an alloy of copper and tin, with a minute proportion of a.r.s.enic (after the example of Newton[318]), and were remarkable for perfection of figure and brilliancy of surface.

The capabilities of the Newtonian plan were developed still more fully--it might almost be said to the uttermost--by the enterprise of an Irish n.o.bleman. William Parsons, known as Lord Oxmantown until 1841, when, on his father's death, he succeeded to the t.i.tle of Earl of Rosse, was born at York, June 17, 1800. His public duties began before his education was completed. He was returned to Parliament as member for King's County while still an undergraduate at Oxford, and continued to represent the same const.i.tuency for thirteen years (1821-34). From 1845 until his death, which took place, October 31, 1867, he sat, silent but a.s.siduous, in the House of Lords as an Irish representative peer; he held the not unlaborious post of President of the Royal Society from 1849 to 1854; presided over the meeting of the British a.s.sociation at Cork in 1843, and was elected Vice-Chancellor of Dublin University in 1862. In addition to these extensive demands upon his time and thoughts, were those derived from his position as practically the feudal chief of a large body of tenantry in times of great and anxious responsibility, to say nothing of the more genial claims of an unstinted hospitality.

Yet, while neglecting no public or private duty, this model n.o.bleman found leisure to render to science services so conspicuous as to ent.i.tle his name to a lasting place in its annals.

He early formed the design of reaching the limits of the attainable in enlarging the powers of the telescope, and the qualities of his mind conspired with the circ.u.mstances of his fortune to render the design a feasible one. From refractors it was obvious that no such vast and rapid advance could be expected. English gla.s.s-manufacture was still in a backward state. So late as 1839, Simms (successor to the distinguished instrumentalist Edward Troughton) reported a specimen of crystal scarcely 7-1/2 inches in diameter, and perfect only over six, to be unique in the history of English gla.s.s-making.[319] Yet at that time the fifteen-inch achromatic of Pulkowa had already left the workshop of Fraunhofer's successors at Munich. It was not indeed until 1845, when the impost which had so long hampered their efforts was removed, that the optical artists of these islands were able to compete on equal terms with their rivals on the Continent. In the case of reflectors, however, there seemed no insurmountable obstacle to an almost unlimited increase of light-gathering capacity; and it was here, after some unproductive experiments with fluid lenses, that Lord Oxmantown concentrated his energies.

He had to rely entirely on his own invention, and to earn his own experience. James Short had solved the problem of giving to metallic surfaces a perfect parabolic figure (the only one by which parallel incident rays can be brought to an exact focus); but so jealous was he of his secret, that he caused all his tools to be burnt before his death;[320] nor was anything known of the processes by which Herschel had achieved his astonishing results. Moreover, Lord Oxmantown had no skilled workmen to a.s.sist him. His implements, both animate and inanimate, had to be formed by himself. Peasants taken from the plough were educated by him into efficient mechanics and engineers. The delicate and complex machinery needed in operations of such hairbreadth nicety as his enterprise involved, the steam-engine which was to set it in motion, at times the very crucibles in which his specula were cast, issued from his own workshops.

In 1827 experiments on the composition of speculum-metal were set on foot, and the first polishing-machine ever driven by steam-power was contrived in 1828. But twelve arduous years of struggle with recurring difficulties pa.s.sed before success began to dawn. A material less tractable than the alloy selected, of four chemical equivalents of copper to one of tin,[321] can scarcely be conceived. It is harder than steel, yet brittle as gla.s.s, crumbling into fragments with the slightest inadvertence of handling or treatment;[322] and the precision of figure requisite to secure good definition is almost beyond the power of language to convey. The quant.i.ties involved are so small as not alone to elude sight, but to confound imagination. Sir John Herschel tells us that "the _total_ thickness to be abraded from the edge of a spherical speculum 48 inches in diameter and 40 feet focus, to convert it into a paraboloid, is only 1/21333 of an inch;"[323] yet upon this minute difference of form depends the clearness of the image, and, as a consequence, the entire efficiency of the instrument. "Almost infinite,"

indeed (in the phrase of the late Dr. Robinson), must be the exact.i.tude of the operation adapted to bring about so delicate a result.

At length, in 1839, two specula, each three feet in diameter, were turned out in such perfection as to prompt a still bolder experiment.

The various processes needed to insure success were now ascertained and under control; all that was necessary was to repeat them on a larger scale. A gigantic mirror, six feet across and fifty-four in focal length, was accordingly cast on the 13th of April, 1842; in two months it was ground down to figure by abrasion with emery and water, and daintily polished with rouge; and by the month of February, 1845, the "leviathan of Parsonstown" was available for the examination of the heavens.

The suitable mounting of this vast machine was a problem scarcely less difficult than its construction. The shape of a speculum needs to be maintained with an elaborate care equal to that used in imparting it. In fact, one of the most formidable obstacles to increasing the size of such reflecting surfaces consists in their liability to bend under their own weight. That of the great Rosse speculum was no less than four tons.

Yet, although six inches in thickness, and composed of a material only a degree inferior in rigidity to wrought iron, the strong pressure of a man's hand at its back produced sufficient flexure to distort perceptibly the image of a star reflected in it.[324] Thus the delicacy of its form was perishable equally by the stress of its own gravity, and by the slightest irregularity in the means taken to counteract that stress. The problem of affording a perfectly equable support in all possible positions was solved by resting the speculum upon twenty-seven platforms of cast iron, felt-covered, and carefully fitted to the shape of the areas they were to carry, which platforms were themselves borne by a complex system of triangles and levers, ingeniously adapted to distribute the weight with complete uniformity.[325]

A tube which resembled, when erect, one of the ancient round towers of Ireland,[326] served as the habitation of the great mirror. It was constructed of deal staves bound together with iron hoops, was fifty-eight feet long (including the speculum-box), and seven in diameter. A reasonably tall man may walk through it (as Dean Peac.o.c.k once did) with umbrella uplifted. Two piers of solid masonry, about fifty feet high, seventy long, and twenty-three apart, flanked the huge engine on either side. Its lower extremity rested on a universal joint of cast iron; above, it was slung in chains, and even in a gale of wind remained perfectly steady. The weight of the entire, although amounting to fifteen tons, was so skilfully counterpoised, that the tube could with ease be raised or depressed by two men working a windla.s.s. Its horizontal range was limited by the lofty walls erected for its support to about ten degrees on each side of the meridian; but it moved vertically from near the horizon through the zenith as far as the pole.