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## CHAPTER III.

NEWTON SUCCEEDS BARROW IN THE LUCASIAN CHAIR — HYPERBOLIC LENSES PROPOSED BY DESCARTES AND OTHERS — OPINIONS OF DESCARTES AND ISAAC VOSSIUS ON COLOURS — NEWTON DISCOVERS THE COMPOSITION OF WHITE LIGHT, AND THE DIFFERENT REFRANGIBILITY OF THE RAYS THAT COMPOSE IT — HAVING DISCOVERED THE CAUSE OF THE IMPERFECTION OF REFRACTING TELESCOPES, HE ATTEMPTS THE CONSTRUCTION OF REFLECTING ONES — CONSTRUCTS A SECOND REFLECTING TELESCOPE IN 1668, WHICH IS EXAMINED BY THE ROYAL SOCIETY, AND SHEWN TO THE KING — DISCUSSIONS RESPECTING THE GREGORIAN, NEWTONIAN, AND CASSEGRAINIAN TELESCOPE — JAMES GREGORY THE INVENTOR OF THE REFLECTING TELESCOPE — ATTEMPTS TO CONSTRUCT ONE — NEWTON MAKES A SPECULUM OF SILVERED GLASS — GLASS SPECULA BY SHORT IN 1730, AND AIRY IN 1822 — HADLEY CONSTRUCTS TWO FINE REFLECTING, TELESCOPES — TELESCOPES BY BRADLEY, MOLYNEUX, AND HAWKSBEE — SHORT'S REFLECTING TELESCOPES WITH METALLIC SPECULA — MAGNIFICENT TELESCOPE OF SIR WM. HERSCHEL WITH A FOUR FEET SPECULUM — MUNIFICENCE OF GEORGE III. — ASTRONOMICAL DISCOVERIES OF SIR WM. HERSCHEL — TELESCOPES OF SIR J. HERSCHEL AND MR. RAMAGE — GIGANTIC TELESCOPE OF THE EARL OF ROSSE WITH A SIX FEET SPECULUM — PROGRESS OF TELESCOPIC DISCOVERY — PROPOSAL TO SEND A FINE TELESCOPE TO A SOUTHERN CLIMATE.

IN 1669, when Dr. Barrow had resolved to devote himself to the studies and duties of his profession, he resigned the Lucasian Professorship of Mathematics, in favour of Newton. His appointment took place on the 29th October, and we may now consider him as having entered on that brilliant career of discovery, the history of which will form the subject of some of the following chapters. It had been long known to every writer on optics, and to every practical optician, that lenses with spherical surfaces, <38> such as those now in common use, did not give distinct images of objects. This indistinctness was believed to arise solely from their spherical figure, in consequence of which the rays which passed through the marginal or outer parts of the lens were refracted to a focus nearer the lens than those which passed through its central parts. The distance between these foci was called the spherical aberration of the lens, and various methods were suggested for diminishing or removing this source of imperfection. Descartes[1] had shewn that hyperbolic lenses refracted the rays of light to a single focus, and we accordingly find the early volumes of the Philosophical Transactions filled with schemes for grinding and polishing lenses of this form. Newton had made the same attempt, but finding that a change of form produced a very little change in the indistinctness of the image, he thought that the defect of lenses, and the consequent imperfection of telescopes, might arise from some other cause than the imperfect convergency of the incident rays to a single point. This happy conjecture was speedily confirmed by the brilliant discovery of the different refrangibility of the rays of light, — a discovery which has had the most extensive applications to every branch of science, and (what is very rare in the history of inventions) one to which no other person has made the slightest claim.

No plausible conjecture, even, had been formed by the predecessors of Newton respecting the nature and origin of colours. Descartes believed them to be a modification of light depending on the direct or rotatory motion of its particles. Grimaldi, Dechales, and others regarded them as arising from different degrees of rarefraction and condensation of light. Gregory defines colour to be the hue <39> (tinctura) Of igneous corpuscles emerging from radiant matter,[2] and we have already seen that the views of Barrow on this subject were equally absurd. In recounting the opinions of preceding writers, Newton alleges that in all of them the colour is supposed not to be innate in light, but produced by the action of the bodies which reflect or refract it. This, however, is not strictly true, as Isaac Vossius in a dissertation which Newton probably never saw, distinctly maintains that all the colours exist in light itself, or to use another of his expressions, that all light carries its colours along with it.[3] This, however, was a mere conjecture, which cannot be regarded as in any way anticipating the great discovery of Newton, "that the modification of light from which colours take their origin, is innate in light itself, and arises neither from reflection, nor refraction, nor from the qualities or any other conditions of bodies whatever, and that it cannot be destroyed or in any way changed by them."

After our author had purchased his glass prism at Stourbridge Fair, he made use of it in the following man <40> ner. Having made a hole H in his window-shutter SHT, and darkened the room, he admitted a ray of the sun's light RR, which after refraction at the two surfaces AC, BC of the prism ABC, exhibited on the opposite wall MN what is called the Solar or Prismatic Spectrum. This spectrum was an elongated image of the sun about five times as long as it was broad, and consisted of seven different colours, Red, Orange, Yellow, Green, Blue, Indigo, and Violet. "It was at first," says Newton, "a very pleasing divertisement to view the vivid and intense colours produced thereby ;" but this pleasure was immediately succeeded by surprise at various phenomena which were inconsistent with the received laws of refraction, The "extravagant disproportion between the length of the spectrum and its breadth," excited him to a more than ordinary curiosity of examining from whence it might proceed. He could scarcely think that the various thickness of the glass, or the termination with shadow or darkness could have any influence on light to produce such an effect; yet he thought it not amiss first to examine these circumstances, and he therefore tried what would happen by transmitting light through parts of the glass of different thickness, or through holes in the window of different sizes, or by setting the prism without, (on the left hand of ST,) so that the light might pass <41> through it and be refracted before it was terminated by the hole; but he found none of these circumstances material. The fashion of the colours was in all these cases the same.

Newton then suspected that by some unevenness of the glass or other accidental irregularity, the colours might be thus dilated. In order to try this he took another prism BCD, and placed it in such a manner that the light passing through them both might be refracted contrariwise, and thus returned by BCD into the path RRW, from which the first prism ABC had diverted it, for by this means he thought that the regular effects of the prism ABC would be destroyed by the second prism BCD, and the irregular ones more augmented by the multiplicity of refractions. The result was, that the light which by the first prism was diffused into an oblong form MN, was reduced by the second prism into a circular one, W with as much regularity as when it did not pass through them, so that whatever was the cause of the length of the image MN, it did not arise from any irregularity in the prism.

Sir Isaac next proceeded to examine more critically the effect that might be produced by the difference in the angles of incidence, at which rays from different parts of the sun's disc fell upon the face AC of his prism, and for this purpose he measured the lines and angles belonging to the spectrum MN, and obtained the following results :

 Distance of MN from the hole H, 22 feet. Length of MN, $13\frac{1}{4}$ inches. Breadth of MN, $2\frac{5}{8}$ " Diameter of the hole H, $0\frac{1}{4}$ " Angle of WR with the middle of MN, 44° 56′. Angle ABC of the prism, 63° 12′. Refractions at R and R', 54° 4′.
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"Now, subducting the diameter of the hole from the length and breadth of the image, there remains 13 inches in the length and $2\frac{3}{8}$ inches in the breadth comprehended by those rays which passed through the centre of the hole, and consequently the angle of the hole which that breadth subtended was about 31′, answerable to the sun's diameter; but the angle which its length subtended was more than five such diameters, namely, 2° 49′."

With the refractive power of the prism, which he found to be 1.55, he found the refractions of two rays proceeding from opposite parts of the sun's disc, so as to differ 31 minutes in their obliquity, to be such as to comprehend an angle of 31 or 32 minutes.

Although Newton could not doubt the correctness of the law of the Sines on which these calculations were founded, yet "his curiosity caused him again to take his prism, and satisfy himself by direct experiment that even a motion of the prism about its axis of four or five degrees, did not sensibly change the position of the spectrum MN on the wall, so that "there still remained some other cause to be found out," from which the spectrum could subtend an angle of 2° 49'.

Having set aside all these explanations of the length of his spectrum, Newton hazarded the strange suspicion that the rays after passing through the prism "might move in curve lines, and according to their more or less curvity lead to different parts of the wall," and "it increased his suspicion," he adds, "when he remembered that he had often seen a tennis-ball struck with an oblique racket describe such a curve line. In this case a circular and a progressive motion being communicated to it by that stroke, its parts on that side where the motions conspire, must press and beat the contiguous air more <43> violently than on the other, and there excite a reluctancy and reaction of the air proportionally greater. And for the same reason, if the rays of light should possibly be (composed of) globular bodies, and by their oblique passage out of one medium into another acquire a circulating motion, they ought to feel the greater resistance from the ambient ether on that side where the motions conspire, and thence be continually bowed to the other. But notwithstanding this plausible ground of suspicion, when I came to examine it, I could observe no such curvity in them. And besides (which was enough for my purpose) I observed that the difference betwixt the length of the image, and the diameter of the hole through which the light was transmitted, was proportional to their distance."

Having. thus gradually removed these different hypotheses, or suspicions, as Newton calls them, he was led to the experimentum crucis for determining the true cause of the elongation of the spectrum MN. He placed a board with a hole in it behind the face BC of the prism, and close to it, so that he could transmit through the hole any one of the colours in MN, and keep back the rest. When the hole was near C, for example, no other rays but the red fell on the wall at N. He then placed behind the red space at N another board with a hole in it, and behind this board he placed another prism, so as to receive the red light at N, which passed through the hole in the board. He then turned round the first prism ABC, so as to make all the colours pass successively through the two holes, and he marked their places on the wall. From the variation of these places he saw that the red rays at N were less refracted by the second prism than the orange rays, the orange less than the yellow, and so on, the violet being more refracted than all the rest. Hence he arrived <44> at the grand conclusion, that light was not homogeneous, but consisted of rays of different refrangibility.

We have given this full account of Newton's mode of investigation, in order to shew the cautious manner in which he proceeded; and were it not for the inconceivable stupidity of the men who called in question his results, we should have considered all his suspicions and precautions unnecessary, and adopted the opinion of Arago, that the compound nature of white light was clearly involved in the very phenomenon of the prismatic spectrum, and that the words in which Newton stated it as a conclusion, were "nothing else than a literal description or translation of that familiar experiment."[4]

Having established this important truth Newton immediately perceived that the different refrangibility of the rays of light was the real cause of the imperfection of refracting telescopes. If LL is a convex lens receiving rays SL, SL, the violet rays in the white ray SL will be refracted in the line LV to V, the yellow rays to Y, and the red rays to R, forming a violet image of the sun, or any other object from which the white light proceeds at the point V, a yellow image at Y, and a red one at R, images of intermediate colours being formed at interme <45> diate points between V and R. If this image is received on a sheet of white paper at V, or Y, or R, it will be exceedingly indistinct, and tinged with these different colours. Newton found that the space VR, which is called the chromatic aberration, or the aberration of colour, was in glass the fiftieth part of the diameter LL of the lens, so that in lenses about six inches in diameter, such as those used in the telescopes about 150 feet long, of Campani, Divini, and Huygens, the space VR would be about .17 of an inch. Hence if LL be the object glass of a telescope directed to any luminous body, and MM an eye-glass through which the eye sees magnified the image or picture of the body between V and R, it cannot see distinctly all the different images of the body formed there. If it is adjusted to see distinctly the yellow image at Y as it is in the figure, it will not see distinctly either the red or the violet images, nor indeed any but the yellow, and that very imperfectly, as it is mixed up with hazy images of all the other colours, producing great confusion and indistinctness of vision.

As soon as Sir Isaac saw this result of his discovery, he left off his "glass-works," as he called his attempts to improve the refracting telescope, and, in the autumn of 1668, constructed the little reflecting telescope which we have already described. The success of this experiment, small as it was, inspired Newton with fresh zeal, and, though his mind was now occupied with his optical discoveries, with the elements of his method of fluxions, and with his speculations on gravity, yet, with all the ardour of youth, he set himself to the task of executing another reflecting telescope with his own hands. This telescope, of which we have given a drawing in the annexed figure, was a better one than the first; and, we presume from <46> its not being much superior either to the first, or to the one executed by his colleague, he allowed it to lie by him for several years. The existence of these telescopes having become known to some of the members of the Royal Society, Newton was requested to send his instrument to that learned body. This telescope consisted of a concave <47> metallic speculum, the radius of curvature of which was $12\frac{2}{3}$ or 13 inches, so that "it collected the sun's rays at the distance of $6\frac{1}{3}$ inches." The rays reflected by the speculum were received upon a plane metallic speculum inclined 45° to the axis of the tube, so as to reflect them to the side of the tube in which there was an aperture to receive a small tube with a plano-convex eye-glass, whose radius was one-twelfth of an inch, by means of which the image formed by the speculum was magnified 38 times.

Newton did not hesitate to obey the request of the Royal Society, and it was accordingly sent, and we believe presented to that distinguished body near the end of 1671. It was also shown to the King, and a description of it published in the Philosophical Transactions.[5] The instrument itself is carefully preserved in the Library of the Royal Society, with the inscription, —

"THE FIRST REFLECTING TELESCOPE INVENTED BY SIR ISAAC NEWTON, AND MADE WITH HIS OWN HANDS."

Previous to March 16, 1672, a Fellow of Trinity College had made a similar telescope of nearly the same size, which Newton found to "magnify more, and also more distinctly," than a six feet refractor, and which he considered better than his own. A description of Newton's instrument in Latin was drawn up and corrected by Newton, and when signed by Lord Brouncker, Wren, and Hooke, was sent to Huygens, who expressed his approbation of it, and suggested the propriety of giving the concave speculum a parabolic form. Various observations were made upon the instrument, particularly by Monsieur Auzout and Monsieur Denys; and Monsieur Berecé claimed for M. Cassegrain the invention of a telescope which <48> he considered "almost like Newton's," and "more ingenious.[6] Newton replied to this communication, and acknowledging that he had been acquainted with the telescope proposed by Gregory before he had contrived his own, he points out the superiority of the Gregorian to the Cassegrainian form, and of his own to both. This letter led to a little amiable controversy between Gregory and Newton on the merits of their reflecting telescopes, in which neither of them gained the victory.[7]

Newton's occupations were at this period too numerous, and his time too valuable to be spent in mechanical labour; and he therefore never resumed the construction of reflecting telescopes. The Royal Society, however, employed a London optician of the name of Cox to execute a Newtonian reflector, with a speculum whose focal length was no less than four feet, but he failed in polishing the speculum, and though Sir Isaac himself contemplated the construction of another instrument, he seems to have wholly abandoned the attempt, and to have be <49> queathed to another age the honour of making his telescope an instrument of discovery. The want of a good material for the specula seems to have been the difficulty which perplexed the optician; and it would appear from the following observations of Newton himself, that the specula for the instrument, ordered by the Royal Society, were to be made of a new material. "You will gratify me much," says he in a letter to Oldenburg, "by acquainting me with the particular dimensions, fashion, and success of the four feet tube, which, I presume, Mr. Cox by this time hath finished. And to inform myself of the advantages of the steely matter which is made use of, you will much oblige me if you can procure me a fragment of it. I suppose it is made by melting steel with a little antimony, perhaps without separating the sulphureous from the metalline part of that mixture. And so though it may be very hard, and capable of a good polish, yet I suspect whether it be so strongly reflective as a mixture of other metals. I make this inquiry, because if I should attempt any thing farther in the fabric of the telescope, I would first inform myself of the most advantageous materials. On which account, also, you would farther oblige me if you can inquire whether Mr. Cox, or any other artificer, will undertake to prepare the metals, glass, tube, and frame of a, four feet telescope, and at what rates he will do it, so that there may remain nothing for me to do but to polish the metals. A gross account of this will at present suffice, until I send you a particular design of the fabric of the instrument, if I resolve upon it."[8]

Although Newton did receive through Oldenburg the information he requested from Mr. Cox,[12] yet he never availed himself of it in proceeding any farther with metallic reflectors. In consequence, however, of Gregory having suggested to him the use of glass specula silvered on the back for burning glasses, and shown how to make <52> the foci of each surface coincident, Newton proposed, we believe in 1678, to substitute these specula instead of metallic ones in the reflecting telescope. In this manner he attempted to make a telescope four feet long, and with a magnifying power of 150; but though the glass was wrought by a London artist, and seemed well-finished, yet, when it was quicksilvered on its convex side, it exhibited all over the glass innumerable inequalities, which rendered every object indistinct. He expresses, however, his conviction, that nothing but good workmanship is wanting to perfect such telescopes, and he recommends their consideration "to the curious in figuring glasses" This recommendation remained unnoticed for upwards of fifty years. At last Mr. James Short, a Scotch artist of consummate skill, executed, about the year 1730, no fewer than six reflecting telescopes, with glass specula, three of which were fifteen inches, and three nine inches in focal length; but some of them turned out useless from the veins in the glass. Maclaurin,[13] who, with one of nine inches, could read the Philosophical Transactions very easily at the distance of 130 feet, informs us that they were excellent instruments. Short, however, found that their light was fainter than he expected, and from this cause, combined with the difficulty of finishing them, he afterwards limited himself to the use of metallic specula.[14]

The subject of glass specula was resumed in 1822 by Mr. Airy, one of the distinguished successors of Newton in the Lucasian chair. Having demonstrated that the aberration both in figure and colour might be corrected <53> in these instruments, he executed more than one; but though the result of the experiment was such as to excite hopes of ultimate success, the construction of such an instrument is still a desideratum in practical science.

Notwithstanding these failures, we would not discourage the young artists of the present day from endeavouring to surmount the difficulties experienced by their predecessors. Discs of glass can now be obtained entirely free of veins and, what is of great importance, instead of coating the convex surface with a plate of mercury and tin, which reflects even less light than speculum metal, we can now, by the electrotype, deposit pure silver on the glass, and give it a reflective power far surpassing that of any other metal.

Such is a brief history of the attempts which were made by Newton and Gregory to construct reflecting telescopes. They were certainly far from being successful; nor were their contemporaries more fortunate, though guided by the light of their experience.

After the lapse of fifty years, however, and several years before his death, Sir Isaac had the satisfaction of seeing a Newtonian telescope, six feet long, mounted upon a commodious stand, and capable of exhibiting some of the most interesting phenomena in the heavens. A Gregorian telescope, of an inferior size, was executed with similar success, and from that time the art of making telescopes with metallic reflectors was gradually brought to perfection. The history of these improvements, and of the grand discoveries in astronomy to which they led, would of itself form an interesting volume. We shall endeavour, in a few pages, to present it to our readers.

Mr Hadley himself and others likewise saw the preceding phenomena together with the belts of Saturn, and the first and second satellites of Jupiter, as bright spots on the body of the planet.[18]

Hitherto no optician but Mr. Hawksbee had ventured to construct these instruments for sale. He executed a good one of about $3\frac{1}{2}$ feet in focal length,[22] and other two of six feet and twelve feet, and he was the first person, as Molyneux informs us, "who had attempted it without the assistance of a fortune, which could well bear the disappointment."

Having acquired by his own experience and Mr. Hadley's instructions, a sufficient knowledge of the art, Mr. Molyneux communicated the whole process[23] to Mr. Edward Scarlet, his Majesty's optician, and to Mr. Hearne, a mathematical instrument maker, and both these artists attained to such perfection in constructing them, that they manufactured them for public sale. In this way the Reflecting Telescope came into general use, and, principally in the Gregorian form, it has been an article of trade with every regular optician.

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While the English opticians, with the aid of Molyneux and Hadley, were thus practising the new art of grinding, and polishing specula, Mr. James Short of Edinburgh, without any such aid, was devoting to the subject all the energies of his youthful mind. In the year 1732, and in the 22d year of his age, he began his labours; and to such perfection did he carry the art of grinding and polishing metallic specula, and of giving them the true parabolic figure, that with a telescope of 15 inches in focal length, he and Mr. Bayne, Professor of Law in the University of Edinburgh, read the Philosophical Transactions at the distance of 500 feet, and several times, particularly on the 24th of November and the 7th of December, 1734, they saw the five satellites of Saturn together, an achievement beyond the reach of Hadley's six feet telescope. Mr. Short had constructed several instruments, 9, 6, 4, and $2\frac{6}{10}$ inches in focal length. With those four inches long he saw the satellites of Jupiter very well, and read in the Philosophical Transactions at the distance of 125 feet. With the six inch ones he read at the distance of 160 feet, and with the nine inch ones at the distance of 160 feet. The celebrated Colin Maclaurin compared one of the six inch ones with one of the best London ones of $9\frac{3}{10}$ inches, and found that it exceeded it in brightness, distinctness, and magnifying power. It surpassed also another London one, $11\frac{1}{2}$ inches in focal length.[24] After Short had established himself in London in 1742, he received £630 for a twelve feet reflector from Lord Thomas Spencer. In 1752 he executed one for the King of Spain for £1200; and a short time before his death, which took place in 1768, he finished the specula of the magnificent telescope which was mounted equatorially for the Observatory of Edinburgh, by his brother <58> Thomas Short. The King of Denmark offered twelve hundred guineas for this instrument, through which we have often seen the leading celestial phenomena, but not till the large speculum had been greatly injured in consequence of having been repolished by an inferior artist.[25]

Notwithstanding these great improvements on the Reflecting Telescope, no discovery of importance had yet been achieved by them. The ordinary refractors of Huygens, and those of Campani in the hands of Cassini, though they laboured under all the imperfections of coloured light, had made the latest discoveries in the heavens; and nearly three quarters of a century had elapsed without any extension of our knowledge of the Solar and Sidereal Systems. This, however, was only one of those stationary intervals during which human genius holds its breath, in order to take a new and a loftier flight. The power of the Refracting Telescope, extended to the unmanageable length of above two hundred feet, had been strained to the very utmost, and the Reflectors, vigorous and promising in their infancy, were about to attain an efficiency and magnitude which the most sanguine astronomer had never ventured to anticipate. It was reserved for Sir William Herschel and the Earl of Rosse to accomplish this great work, and by the construction of telescopes of gigantic size to extend the boundaries of the Solar System — to lay open the hitherto unexplored recesses of the sidereal world, and to bring within the grasp of reason those nebular regions to which imagination had not ventured to soar.

Anxious to observe with his own eyes the wonders of <59> the planetary system, and, fortunately for science, unable to purchase a telescope for himself, Sir William Herschel resolved, in l774, to construct one with his own hands. With this instrument, which was a Newtonian reflector of five feet, he saw distinctly the ring of Saturn and the satellites of Jupiter. Dissatisfied with its performance, he afterwards executed two hundred specula of seven feet focal length, one hundred and fifty of ten feet, and above eighty of twenty feet ! In 1781 he began a thirty feet aerial reflector, with a speculum three feet in diameter, but as it was cracked in the operation of annealing, and as another of the same size was lost in the fire from a failure in the furnace, his hopes were disappointed. In minds like his, however, disappointment is often a stimulus to higher achievements, and the double accident which befell his specula suggested, no doubt, the idea of making a still larger instrument, and of obtaining pecuniary aid for its accomplishment. He accordingly conveyed, through Sir Joseph Banks, to the King his intention to execute such a telescope, and his Majesty, with the munificent spirit of a great sovereign, instantly offered to defray the whole expense of its construction. Encouraged by this noble act of liberality, Sir William Herschel began in 1785, and completed in 1789, his gigantic, telescope, forty feet in focal length, with a speculum forty-seven and a half inches in diameter! Its tube about forty feet long and five wide, was made of iron, and the observer, suspended in a moveable seat at the mouth of it, examined, with what is called the front view, the celestial objects to which it was directed. This noble instrument, now dismantled, stood in the lawn of Sir William Herschel's house, and some of our readers may remember, like ourselves, its extraordinary aspect when visiting the great astronomer <60> himself, or resting in the Crown Hotel at Slough, or journeying on their way to Windsor.

It is due to the memory of George III., that the friends of science should cherish it with respect and gratitude. By enabling Sir William Herschel to construct his colossal tube, and to spend the whole of his time in applying it to the heavens, he was entitled to share in the glory of his discoveries; and we owe it to historical truth to say, that none of the sovereigns who either preceded or followed him have an equal claim on the homage of astronomers. If, in his imperial rule, he sometimes transcended the limits of constitutional government, let us remember that he left the throne more secure and glorious than he found it. If he ventured, on some occasions, to thwart the counsellors of his choice, we may find some apology for the exercise of a high prerogative in the factious character of the age, and in the acknowledged incapacity of his advisers; — and if he lost a transatlantic empire by persisting to levy tribute from its people, he followed the advice of distinguished counsellors, and was but the instrument of a higher power in establishing a mighty nation veined with Saxon blood, and nerved with British spirit, — destined to give lessons of civilisation to the Eastern World — to afford a home to science unpatronized — to religion in persecution, and to patriotism in exile.

Stimulated by such patronage, the genius and perseverance which created instruments so transcendent in magnitude, were not likely to be baffled in their practical application. In the examination of the starry heavens, the ultimate object of his labours, Sir William Herschel exhibited the same exalted qualifications; and in a few years he rose from the level of humble life to the enjoyment of a name more glorious than that of the sages and <61> warriors of antiquity, and as enduring as the objects with which it will be for ever associated. Nor was it in the ardour of the spring of life that these triumphs were achieved. He had reached the middle of his appointed course before his career of discovery began, and it was in the autumn and winter of his days that he reaped the full harvest of his glory. The discovery of a new planet at the verge of the Solar System, was the first trophy of his skill, and new double and multiple stars, and new nebulæ and groups of celestial bodies, were added in hundreds to the system of the universe. The spring tide of knowledge, which was thus let in upon the human mind, continued for a while to spread its waves over Europe, but when it sank to its ebb in England, there was no other bark left upon the strand but that of the Deucalion of science, whose home had been so long upon its waters.[26]

When Sir William Herschel's great telescope was taken down in 1822, a telescope of 20 feet in focal length, and with an aperture of $18\frac{1}{4}$ inches, was erected in its place by his son, Sir John Herschel. This instrument, with three mirrors of the same size, was carried to the Cape of Good Hope, and it was with it that Sir John made those valuable observations which have added so greatly to our knowledge of Sidereal Astronomy.

About the same time, the late Mr. John Ramage, a merchant in Aberdeen, devoted much of his attention to the construction of large Newtonian reflectors. He ground and polished specula of $13\frac{1}{2}$, 15, and 21 inches in diameter. One of these was erected at the Royal Observa <62> tory of Greenwich in 1820,[27] with a focal length of 25 feet, and a speculum 15 inches in diameter; — another of the same size at Sir John Ross's Observatory, near Stranraer; — and the large speculum of 21 inches, is, we believe, in the Observatory of Glasgow.[28]

The long interval of half a century seems to be the period of hybernation during which the telescopic mind rests from its labours, in order to acquire strength for some great achievement: Fifty years elapsed between the dwarf telescope of Newton and the large instruments of Hadley: Other fifty years rolled on before Sir William Herschel constructed his magnificent telescope; and fifty years more passed away before the Earl of Rosse produced that colossal instrument which has already achieved such brilliant discoveries.

This distinguished nobleman began his experiments so early as 1828, and he ground and polished specula fifteen inches, two feet and three feet in diameter, before he commenced the Herculean attempt of executing a speculum six feet in diameter, and with a focal length of fifty feet. The speculum was cast on the 13th April 1842, ground in 1843, polished in 1844, and, in February 1845, the telescope was ready to be tried. The focal length of the speculum is fifty-four feet. It weighs four tons, and, with its supports, it is seven times as heavy as the four feet speculum of Sir William Herschel. The speculum is placed in one of the sides of a cubical wooden-box s, Fig.6, about eight feet wide, and to the opposite end of this box is fastened the tube, which is about fifty feet long, eight <63> feet in diameter in the middle, but tapering to seven at the extremities, and furnished with diaphragms $6\frac{1}{2}$ feet in aperture. The tube is made of deal-staves an inch thick, hooped with strong iron clamp rings, and it carries at its upper end, and in the axis of the tube, the small oval speculum A, six inches in its lesser diameter.

The telescope, as shewn in the annexed figure, is established between two lofty castellated piers sixty feet high, and is raised to different altitudes by a strong chain cable B attached to the top of the tube. This cable passes over a pulley T on the frame F down to a windlass shewn at U in Fig. 6, on the ground, which is wrought by two assistants. To the frame F are attached, at X, X, chain guys fastened to the counterweights E, E. The telescope is balanced by these counterweights suspended by chains D, D, which are fixed to the sides of the tube, and pass over large iron pulleys C, C.

To the eastern pier is fixed a strong semicircle of cast <64> iron V, V, about eighty-five feet in diameter. The telescope is connected with this circle by a strong racked bar W with friction-rollers attached to the tube by wheel-work, so that by means of a handle near the eye-piece, the observer can move the telescope along the bar on either side of the meridian to the distance of an hour for an equatorial star.

On the western pier are erected the stairs and galleries for the observers. The first gallery, shewn at H, H below the tube, Fig. 5, commands an altitude of 42°. It is a light but strong framing of wood, which slides between two ladders I, I, fixed to the southern face of the piers. It is counterpoised by a weight, and raised to the height required by a windlass K. Upon its upper plane is a railway upon <65> which the observing gallery L can be moved about 24 feet east and west by means of two wheels turned by a winch M near the observer. Other three galleries, N, O, P, command all altitudes above 42°, and within 5° of the zenith. They are each carried by two beams Q, Q, which run between pairs of grooved wheels R, R, and these beams, with their respective galleries, are drawn forward when the wheels are turned by a very ingenious piece of mechanism. These galleries hold twelve persons, and strangers are not a little startled when they find themselves suspended, midway between the piers, over a chasm 60 feet deep.[29]

We have enjoyed the great privilege of seeing and using this noble instrument, one of the most wonderful combinations of art and science which the world has yet seen. We have, in the morning, walked again and again, and ever with new delight, along its mystic tube, and, at midnight, with its distinguished architect, pondered over the marvellous sights which it discloses, — the satellites, and belts, and rings of Saturn, — the old and new ring, which is advancing with its crest of waters to the body of the planet, — the rocks, and mountains, and valleys, and extinct volcanoes of the moon, — the crescent of Venus, with its mountainous outline, — the systems of double and triple stars, — the nebulæ and starry clusters of every variety of shape, — and those spiral nebular formations which baffle human comprehension, and constitute the greatest achievement in modern discovery.

Such is a brief description of the gigantic telescope completed by the Earl of Rosse. In order to form a correct idea of its effective magnitude, we must compare it with other instruments, as in the following table, in <66> which the specula are supposed to be square in place of round: —

 Names of Makers. Diameter of Speculum. Area of Surface. Newton, 1 inch, 1 square inch. — 2.37 " 5.6 " Hadley, 4.5 " 20 " — 5 " 25 " Hawksbee, 9 " 81 " Ramage, 21 " 441 " Lassels, 2 feet, 576 " Lord Rosse, 2 " 576 " — 3 " 1296 " Herschel, 4 " 2304 " Lord Rosse, 6 " 5184 "

In looking back on what the telescope has accomplished since the time of Newton, and in reflecting on the vast depths of ether which have been sounded; — on the number of planetary bodies which have been added to our system, and on the extensive fields of sidereal space which have been explored, can we hesitate to believe it to be the Divine plan that man shall yet discover the whole scheme of the visible universe, and that it is his individual duty, as well as his high prerogative, to expound its mysteries and to develop its laws? Over the invisible world he has received no commission to reign, and into its secrets he has no authority to pry. It is over the material and the visible that he has to sway the intellectual sceptre, — it is among the structures of organic and inorganic being that his functions of combination and analysis are to be chiefly exercised. However great have been the achievements of the past, and however magnificent the instruments to which we owe them, the limits of telescopic, vision have not been reached, and space has yet marvellous secrets to surrender. A ten feet reflector will be due to science before the close of the century, <67> and a disc of flint-glass,[30] 29 inches in diameter, awaits the command of some liberal government, or some munificent individual, to be converted into an achromatic telescope of extraordinary power.

In cherishing these sanguine expectations, we have not forgotten that the state of our northern atmosphere must set some limit to the magnifying power of our telescopes. In a variable climate, indeed, the vapours and local changes of temperature, and consequent inequalities of refraction, offer various obstructions to astronomical research. But we must meet the difficulty in the only way in which it can be met. The astronomer cannot summon the zephyrs to give him a cloudless sky, nor command a thunderstorm to clear it. He must transport his telescope to the purer air of Egypt or India, or climb the flanks of the Himalaya or the Andes, to erect his watch-tower above the grosser regions of the atmosphere. In some of those brief yet lucid intervals, when distant objects present themselves in sharp outline and minute detail, discoveries of the highest value might be grasped by the lynx-eyed astronomer. The resolution of a nebula, — the bisection of a double star, — the detection of the smaller planetary fragments; — the details of a planet's ring, — the evanescent markings on its disc, — the physical changes on its surface, and perchance the display of some of the dark worlds of Bessel, might be the revelations of a moment, and would amply repay in national glory the transportation of a huge telescope to the shoulder or to the summit of a lofty mountain.[31]

[1] Dioptrice, cap. viii. ix., 1629.

[2] Optica Promota: Definitiones, 3. Lond. 1663.

[3]

Isaaci Vossii De Lucis Natura et Proprietate, Amstel. 1662. As the opinions of Vossius have not been referred to by any of our historians of science, the following passages may be interesting.

"Primus itaque color, si tamen color dicendus sit, is est albus, pelluciditatem proxime hic accedit. Insunt itaque et lumini omnes colores, licet non semper visibiliter; nempe ut flamma intensa alba et unicolor apparet, eadem si per nebula aut aliud densius corpus spectetur, varios induit colores. Pari quoque ratione, Lux, licet invisibilis ant alba ut sic dicam, si per prisma vitreum, aut aerem roridum transeat, similiter varios colores induit." — P. 6.

"Omnem tamen lucem secum colores deferre et eo colligi potest quod si per lentem vitream, aut etiam per foramen, lumen in obscurum admittatur cubiculum in muro aut linteo remotiore manifeste omnes videantur colores, cum tamen in punctis decussationis radiorum et locis minimum lenti vicinis, nullus color sed purum tantum compareat lumen." — P. 64.

"Quapropter non recte ii sentiunt qui colorem vocant Lumen modificatum." — P. 59.

[4] Phil. Trans., vol. vii., No. 80. Feb. 19, 1672

[5] Phil. Trans., vol. vii., No. 81, p. 4004. March 25, 1672.

[6] See Journal des Savans , 1672, pp. 80 and 121; and Phil. Trans., No. 83, p. 4056, May 20, 1672.

[7] Gregory's Catoptrics, App. 261. In this controversy, Newton never claimed any credit for the invention of a new form of the reflecting telescope, and was certainly surprised at the notice it excited among persons that either were, or ought to have been, acquainted with the previous invention of Gregory. In his letter to Mr. Collins, he speaks in the kindest manner of Gregory. "I doubt not that when Mr. Gregory wrote his Optica Promota, he could have described more fashions than one of these telescopes, and perhaps have run through all the possible cases of them, if he had thought it worth his pains. Because Mr. Cassegrain propounded his supposed invention pompously, as if the main business was the contrivance of these instruments, I thought fit to signify that that was none of his contrivance, nor so advantageous as he imagined. And I have now sent you these farther considerations on Mr. Gregory's answer, only to let you see that I chose the most easy and practicable way to make the first trials. Others may try other ways, nor do I think it material which way these instruments are perfected, so they be perfected. — Dec. 10, 1672." See the Macclesfield Collections, vol. ii. pp. 346, 347, or Newtoni Opera by Horsley, vol. iv. p. 288.

[8] July 13, 1672, in the Macclesfield Collections, vol. ii. p. 333.

[9] Sir Isaac seems to have been the first person who suggested the idea that vision might be rendered indistinct by the collision of the rays when they cross one another at the focus of mirrors or lenses. In speaking of the use of more than one <51> eye-glass in the Gregorian telescope, he states, that "by the iterated decussations of the rays, objects will be rendered less distinct , as is manifest in dioptric telescopes, where two or three eye-glasses are applied to erect the object." — Letter to Collins, Dec. 10, 1672; Macclesfield Collections, vol. ii. p. 344. In the course or some experiments on this subject, I found that the sections of the cone of rays, are never so distinct and well-defined after the rays have crossed as before. — (Treatise on New Phil. Inst., pp. 44 & 193). And Captain Kater, in comparing two equal telescopes, the one Gregorian and the other Cassegrainian, found that the intensity of the light within the focus was nearly double of what it was without the focus. In other experiments, he found the ratio as 1000 to 788 — Phil. Trans., pp. 13, 14. Mr. Tulley, however, in making similar experiments, did not confirm the results obtained by Captain Kater. I have found, in confirmation of these facts, that the negative diffractive fringes produced by rays which do not cross one another before they enter the eye, are more distinct than the positive ones which do cross. — Treatise on Optics, Edit. of 1853, p. 117.

[10] Dr. Hook made several experiments with the speculum executed by Mr. Reeves, and did not find it so bad as Gregory thought. See Newton's letter above referred to.

[11] Letter from Gregory to Collins and Newton, Sept. 26. 1672.

[12] Biog. Brit., Art. Newton, p. 3217.

[13] Smith's Optics, vol. ii. Remarks, p. 80.

[14] Caleb Smith proposed to correct the colour produced by the two refractions, by a concave lens placed between the speculum and the small receiver, or by making the surface of a rectangular glass prism concave. — Phil. Trans. 1739, p. 326.

[15] See Prof. Rigaud's Biographical Account of John Hadley, Esq., pp. 7-11.

[16] Phil. Trans., vol. xxxii. No. 376, March and April, 1723, p. 303.

[17] Phil. Trans., July and August 1723, p. 382.

[18] Gregory's Catoptrics, pp. 250, 285.

[19] Ibid., p. 385

[20] The Hon. Samuel Molyneux and Hadley in Smith's Optics, vol. ii. p. 302, § 782.

[21] Ibid., p. 363, § 913.

[22] This telescope, according to Dr. Smith, was so excellent that it was scarcely inferior to Hadley's of 5 feet $2\frac{1}{2}$ inches in length. It bore a power of 226, as determined by Mr. Hawksbee, Mr. Folkes, and Dr. Jurin. See Smith's Optics; Remarks, p. 79.

[23] This process, drawn up partly by Molyneux, and partly by Hadley, is printed in Dr. Smith's Optics, vol. ii. p. 301.

[24] Maclaurin in Smith's Optics, vol. ii., Remarks, p. 81.

[25] This telescope was removed from the Observatory upon the establishment of the Astronomical Institution, and is, we believe, now lying dismantled in some garret of the city.

[26] For an account of the Decline of Science in England, here alluded to, we refer the reader to Sir John Herschel's Treatise on Sound, to Mr. Airy's Report on Astronomy, in the Report of the British Association for 1833, and to Mr. Babbage's interesting volume, On the Decline of Science. See also Quarterly Review, October 1830, and North British Review, vol. xiv. p. 235.

[27] See Transactions of the Astronomical Society, vol. ii. p. 413.

[28] A fine reflecting telescope, with a speculum two feet in diameter, and a focal length of twenty feet, has been recently constructed by Mr. Lassels, who has made with it several important discoveries within the limits of our own system.

[29] A box containing a second speculum is shewn at Y.

[30] This disc of flint-glass was executed by Messrs. Chance, Brothers, and Company, of the Smethwick Glass-works, and was rewarded with a council medal of the Great Exhibition. — See Reports of the Juries, p. 529.

[31] This proposal, which was first made by the author in September 1844, is likely to be now carried into effect. A committee of the British Association, and of the Royal Society, have, after a careful consideration of the subject, applied to Government for the necessary funds.