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We shall best appreciate the greatness of Lord Rosse's service to astronomy by considering what it was his predecessors left him to complete; and reflecting that, in the completion of their work, he has not only achieved for himself the triumph of constructing this one noble instrument, but shown others the way of repeating the same triumph with unerring certainty and precision.

The telescope is not without its type in nature. The achromatic lenses of the eye are adjusted in a kind of optic instrument, the perfection of which art even now seeks in vain to emulate. Yet, like many other great discoveries, it seems to have been first stumbled on accidentally by a Dutch toy-man. But it is science alone which can use aright the capricious gifts of Fortune. Galileo heard of the Dutchman's toy, and in his hand the little leaden tube of a few inches, with a convex and concave spectacle-glass at either end, became the revealer of the true system of the universe.

τυτθὸν ἑοὶ τὸ βέλεμνον, ἐς αἴθερα δ ̓ ἄχρι φορεῖται.

The splendid dream of Copernicus was no longer mere theory, but the astronomer saw visibly before him earth's sister-worlds revolving in their orbits. The marvellous theatre, which so small and rude an instrument was sufficient to disclose, soon stimulated the zeal of philosophers to improve its powers, and, under the hands of Huygens, Campani, and Cassini, it gradually shot up into a column 140 feet in length. But there were causes limiting the development of the refracting telescope, which science, with all her resources, was unable to remove. Not the least considerable of these arises from the circumstance, that, in enlarging the object glass, we expose it to the inevitable risk of changing its figure by the pressure of its own weight, when supported only by the rim; while a support which should prevent its sinking, without intercepting the observer's view, has hitherto been sought in vain.

The difficulty of dealing with the refracting instrument led Gregory, in 1663, to attempt the construction of a reflecting telescope. He made one speculum of a concave shape, in the figure of a parabola, which was perforated in the centre; and before this he set another speculum, concave also, but elliptic, at the distance of a little more than the sum of their focal lengths. The image of the object, formed behind the larger speculum, was viewed through a magnifying eye-glass placed at the middle of the tube. Gregory's attempt was a failure; but in 1666, Sir Isaac Newton succeeded in constructing the first reflecting telescope on record. He improved on Gregory's plan, by setting the eye-glass in the side of the tube, and dispensing altogether with the awkward hole in the large speculum. This telescope was but six inches long, with an aperture of one inch, yet it proved as serviceable as a refractor of six feet. In 1719, Hadley, under Newton's directions, constructed another reflecting telescope, which, though but six feet long, magnified 100 times; and the manifest superiority of the new instrument soon roused the energy of others to improve upon the idea. The great difficulty was in the preparation of the specula, securing their exact parabolic form, and requisite equability of polish. Of all who, before Herschel, laboured upon this task, the Scottish artist, Short, was undoubtedly the most successful; but, with the niggardly spirit of a tradesman, he kept his secret entirely to himself, and it died with him. Herschel, when his bold spirit prompted him to attempt those giant creations which have made his name immortal, had to rely upon his own skill to prepare the means for that scrutiny of the realms of space upon which his soul was bent. He laboured long upon his appointed task, at his own proper cost and peril, with a zeal and devotion such as none who have not felt the thirst of knowledge can conceive, until, supported by the discerning patronage of George III., he perfected what was long supposed the ne plus ultra of such works-a reflecting telescope of forty feet in length, with a speculum of four feet in diameter. But, through an unhappy neglect, the account (though actually, it seems, prepared) of the processes by means of which such marvellous effects were produced, was never given to the public. Men were deterred from an attempt at repetition by the hazardousness of the costly experiment, and the wonderful telescope of Slough remained without a rival in the world, until Lord Rosse conceived the plan which has enabled him not only to equal, but surpass, that far-famed in

VOL. XXXVI.-NO. CCXI.

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We shall best appreciate the greatness of Lord Rosse's service to astronomy by considering what it was his predecessors left him to complete; and reflecting that, in the completion of their work, he has not only achieved for himself the triumph of constructing this one noble instrument, but shown others the way of repeating the same triumph with unerring certainty and precision.

The telescope is not without its type in nature. The achromatic lenses of the eye are adjusted in a kind of optic instrument, the perfection of which art even now seeks in vain to emulate. Yet, like many other great discoveries, it seems to have been first stumbled on accidentally by a Dutch toy-man. But it is science alone which can use aright the capricious gifts of Fortune. Galileo heard of the Dutchman's toy, and in his hand the little leaden tube of a few inches, with a convex and concave spectacle-glass at either end, became the revealer of the true system of the universe.

τυτθὲν ἐπὶ τὸ βέλεμνον, ἐς αἴθερα δ ̓ ἄχρι φορεῖται.

The splendid dream of Copernicus was no longer mere theory, but the astronomer saw visibly before him earth's sister-worlds revolving in their orbits. The marvellous theatre, which so small and rude an instrument was sufficient to disclose, soon stimulated the zeal of philosophers to improve its powers, and, under the hands of Huygens, Campani, and Cassini, it gradually shot up into a column 140 feet in length. But there were causes limiting the development of the refracting telescope, which science, with all her resources, was unable to remove. Not the least considerable of these arises from the circumstance, that, in enlarging the object glass, we expose it to the inevitable risk of changing its figure by the pressure of its own weight, when supported only by the rim ; while a support which should prevent its sinking, without intercepting the observer's view, has hitherto been sought in vain.

The difficulty of dealing with the refracting instrument led Gregory, in 1663, to attempt the construction of a reflecting telescope. He made one speculum of a concave shape, in the figure of a parabola, which was perforated in the centre; and before this he set another speculum, concave also, but elliptic, at the distance of a little more than the sum of their focal lengths. The image of the object, formed behind the larger speculum, was viewed through a magnifying eye-glass placed at the middle of the tube. Gregory's attempt was a failure; but in 1666, Sir Isaac Newton succeeded in constructing the first reflecting telescope on record. He improved on Gregory's plan, by setting the eye-glass in the side of the tube, and dispensing altogether with the awkward hole in the large speculum. This telescope was but six inches long, with an aperture of one inch, yet it proved as serviceable as a refractor of six feet. In 1719, Hadley, under Newton's directions, constructed another reflecting telescope, which, though but six feet long, magnified 100 times; and the manifest superiority of the new instrument soon roused the energy of others to improve upon the idea. The great difficulty was in the preparation of the specula, securing their exact parabolic form, and requisite equability of polish. Of all who, before Herschel, laboured upon this task, the Scottish artist, Short, was undoubtedly the most successful; but, with the niggardly spirit of a tradesman, he kept his secret entirely to himself, and it died with him. Herschel, when his bold spirit prompted him to attempt those giant creations which have made his name immortal, had to rely upon his own skill to prepare the means for that scrutiny of the realms of space upon which his soul was bent. He laboured long upon his appointed task, at his own proper cost and peril, with a zeal and devotion such as none who have not felt the thirst of knowledge can conceive, until, supported by the discerning patronage of George III., he perfected what was long supposed the ne plus ultra of such works-a reflecting telescope of forty feet in length, with a speculum of four feet in diameter. But, through an unhappy neglect, the account (though actually, it seems, prepared) of the processes by means of which such marvellous effects were produced, was never given to the public. Men were deterred from an attempt at repetition by the hazardousness of the costly experiment, and the wonderful telescope of Slough remained without a rival in the world, until Lord Rosse conceived the plan which has enabled him not only to equal, but surpass, that far-famed in

VOL. XXXVI.-NO, CCXI.

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strument. He was the knight for whom this great adventure was reserved; and all the sciences united to accomplish him with the proper panoply for ensuring success. He it is (to borrow Dr. Robinson's eloquent words) who," by a rare combination of optical science, chemical skill, and practical mechanics, has given us the power of overcoming difficulties which arrested our predecessors, and of carrying to an extent, which even Herschel himself did not venture to contemplate, the illuminating power of this telescope, along with a sharpness of definition scarcely inferior to that of the achromatic." So true is it that all sciences are related, and that the perfection of any one of them requires the development of the rest.

"Alterius sic
Altera poscit opem res et conspirat amice!"

The great difficulty of constructing specula for reflecting telescopes lies partly in the matter and partly in the form. The metal, to make a proper mirror, must be white, with a brilliancy at once high and lasting. These qualities are best ensured by a combination of copper and tin, in the proportion of four equivalents of copper to one of tin. Any departure from this definite combination is sure to be punished by the tarnishing of the compound; and yet the temptations to depart from it are so great that even Herschel himself was forced to yield to them. The metal, when thus compounded, is so brittle that not only a slight blow, but even a sudden increase in temperature, will make it split; and even when debased by a larger mixture of copper, the heat generated by the friction of the tool in grinding has marred all the previous success of the artist, and ruined in a moment the effect of weeks of toil. The casting of large specula in metal of this standard might at first seem hopeless, since the slightest inequality of expansion in cooling must inevitably spoil the work, and Lord Rosse's first device was to attack the enemy in detail. He constructed his speculum piece-meal. His first mirror of three feet was cast in sixteen pieces. Each piece was fixed upon a back of an alloy composed of copper and zinc, in the proportion of 2.75 of the former to one of the latter, which compound has the fortunate property of expanding and contracting in the same degree as the speculum-metal itself. When the soldering and polishing were accomplished, it was found that an available plated speculum was the result, and that, by diminishing the number and size of the joints, the slight imperfections arising from diffraction, occasioned by its piece-meal construction, might be made almost imperceptible. Still these could not be diminished without enlarging the plates, and the plates could not be enlarged without increasing the risk of flaws. The final triumph, therefore remained to be achieved in the casting of a vast solid mirror of this brittle substance, and forcing its coy nature to yield unqualified submission to the behests of science. The great question was, of what to make the mould? Sand, which Edwards had recommended, was found insufficient. The edges of the metal cooling in the mould became solid ere the centre had lost its fluidity. The plates were, therefore, full of flaws, and flew in pieces in the setting. A solid mould of cast-iron was next tried, with a jet of cold water on its lower surface, but this plan cracked the mould itself. The third was nearer the aim― a mould with an under surface of iron and sides of sand. But here a new difficulty arose. The air could not escape through the iron disc, and large holes were left in the metal, thus saved from one imperfection at the cost of another. But, nevertheless, a great step had been made. rgis mèv ögstar' ïwv, rò dì tírgatev InsTo ring. The grand question had resolved itself into the problem of finding an exit for the air, and this troublesome captive was set free at last by employing a bottom of hoop-iron layers, tightly packed together in an iron frame, with their edges up, but smoothed by turning or filing to the proper curvature. The interstices were small enough to retain the metal and suffer the air to escape. Thus, at last, a solid speculum of three feet in diameter was successfully cast. But the casting gives only the rough block, which is yet to be ground and polished into a mirror, and the polishing was hitherto a work regarded with still greater apprehension than the casting. The operation had to be performed with the hand, an instrument which can never be precisely regular in its movements or pressures, especially when repeated often through a long space of time. Lord Rosse's improvement of this part of the process consists in substituting mechanical for human agency. The speculum is made to revolve slowly in a

tank of water, to prevent the extrication of heat by friction, and the polisher is worked on the mirror with long and quick strokes. It is of the same diameter as the speculum, intersected with transverse and circular grooves, not exceeding half an inch of surface, covered, when the polishing is to be effected, with two strata (a hard and soft) of resin and turpentine, smeared over with rouge and water, mixed to about the consistency of cream. The whole machine is worked by steam, and the effect of the grinding is noted by observing the reflection of the dots in the dial of a watch, mounted on a mast at the top of the high tower, in the lowest room of which the grinding is carried on. The tower is, as it were, the tube of a telescope; the watch, the object; and the inchoate speculum, the mirror. Trap-doors in the intervening floors of the tower are thrown open when the observation is required; and when the dots are seen in sharp definition, the grinding is complete. The polishing is effected with perfect certainty and precision in six hours. We have now brought the three-feet speculum to its last polish; but, in completing it, the philosopher saw clearly that the way was opened for a still grander effort—a speculum of six feet in diameter, and a focal length of fifty-three.

Former triumphs made this easy. The great block was but three weeks in the annealing oven, and was polished as speedily as the smaller mirror; but new devices were required for rendering it available in a telescope. It weighs three tons, and, to prevent all risk of bending, is made to rest upon a diffused system of supports, so ingeniously determined on points at their different centres of gravity, as to secure the mirror from being affected by accidental changes. The tube is a pillar forty feet in length, "of deal staves hooped like a cask," seven feet in its diameter. But for supporting this monstrous mass, strong walls on either side (forty-eight feet high on the outer side, and fifth-six on the inner) were found necessary; and its lateral movements are only from one wall to another, so as to command a view, for half-an-hour, at each side of the meridian. On these walls, by strong chains, the counterpoises are hung, whose nice adjustment enables a human arm, by turning a windlass, to command at will the services of this giant minister. The telescope is used as a Newtonian. The image in the great speculum is thrown up on a small mirror, which is observed from an aperture in the side; the spectator standing in a moveable gallery attached to one of the piers, but capable of following the tube in all its revolutions. It might be used also as a Herschelian; but it is judged that in the observation of Nebula (its principal task hitherto), more is gained in the sharp definition of the object (which would be impaired by inclining the great speculum to the incident rays) than is lost in brilliancy by the second reflection.

Let it not be forgotten that, in every step of the vast and elaborate works which we have thus imperfectly described, it was not only Irish genius which directed, but Irish diligence and skill which executed the task. Common Irish labourers, working under his lordship's eye, were found quite adequate to accomplish all, where the nicest precision of mathematical exactness was required at every point; and, curiously enough, as if to make this great scientific monument entirely home manufacture, turf was found the best fuel for melting the metal of the speculum. Would that the climate of our Island were as propitious to Astronomy as its soil! But there seems some unhappy antagonism between heaven and earth, which forbids the permanent green of the one to co-exist with the permanent azure of the other; and the uniform hazy canopy which preserves the verdure of our fields, shuts out too often from the eye of the astronomer those distant worlds which he desires to scan. Still, notwithstanding frequently recurring interruptions, that "broad bright eye," so steadily fixed on its inconstant object, has read enough of the secrets of the heavens to reward all the labours which were required to prepare it for its watch. There is something stern in "the plain tale" by which this truthful reporter has "put down" a number of bold assertions, long listened to with willing ears by semiscientific auditors. Still as the orb of true science makes its way, the clouds of opinion which refract its light through their many-coloured medium, hover round it, and appear to glorify and expand the circumference which they obscure; and to many an eye the luminary itself, when freed from these earthborn vapours, looks as it were "shorn of its beams," and contracts into seeming insignificance. Had Fontenelle lived on to our own days (and he promised

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