steadiness of the image when highly mag- however, doubtless at his instigation, emnified, set a limit to their length. In the ployed a London optician, of the name of present day, when it is easy to construct Cox,* to execute a reflector like Newton's, plane metallic reflectors, ordinary refract-four feet long, but he failed in polishing ing telescopes, of any length-a thousand the mirror, and no further attempt was feet for example-might be brought into made to construct reflecting telescopes, till use by using a dry ditch for their tube, and reflecting the rays of the celestial body along its axis. In this way the most perfect steadiness would be obtained; the object glass would be accessible for the purpose of cleaning it, and the air in the tube and every part of the instrument might be preserved at an uniform temperature.

In the year 1663, when Huygens was occupied with the improvement of refracting telescopes, our countryman James Gregory published an account of the reflecting telescope, to which his name has since that time been attached. It consisted of a concave speculum of a parabolic form, perforated at its centre. In front of it was placed a small concave speculum of an elliptical form, the distance of the two being a little greater than the sum of their focal lengths. The image of a distant object was formed behind the larger speculum, and there magnified by an eye-piece. In 1666, Sir Isaac Newton made a change in the construction of this telescope, by " placing the eye-glass at the side of the tube, rather than at the middle;" and in this way he dispensed with the aperture in the larger speculum. Mr. Gregory failed in the construction of his instrument, probably from the want of the eye-stop, and hence Newton had the honor of being the first person who made a reflecting telescope. It was only six inches long, with a speculum of an inch in aperture. It magnified 40 times, and performed as well as a six foot refractor, showing the satellites of Jupiter and the phases of Venus. In 1671, Newton completed an instrument with a speculum 23 1-8 of an inch in diameter, which was exhibited to the King and to the Royal Society in 1672, and which is now in the library of that Institution, with the inscription

"The First Reflecting Telescope, invented by Sir Isaac Newton, and made with his own hands."

John Hadley, a country gentleman in Essex, and the inventor of Hadley's Quadrant, directed his attention to the subject. This ingenious individual completed one of these instruments in 1719, and presented it to the Royal Society, whose journals for January 12, 1721, contain the following notice of it." Mr. Hadley was pleased to show the Society his reflecting telescope, made according to our President (Newton's) directions in his optics, but curiously executed by his own hand, the force of which was such as to enlarge an object near two hundred times, though the length thereof scarce exceeds six feet; and having shown it he made a present thereof to the Society, who ordered their hearty thanks to be recorded for so valuable a gift." By means of this telescope, Hadley saw the transit of Jupiter's satellites, and their shadows on the disc of the planet; the division in Saturn's ring, and the shade of the planet cast upon it; but he was not able to distinguish more than three of the satellites. Dr. Pound and Dr. Bradley, who repeatedly observed with it, found that it represented objects" as distinct, though not altogether so clear and bright" as the telescope of Huygens.t

The celebrated Samuel Molyneux and Dr. Bradley, were instructed in the art of grinding and polishing metallic specula, by Mr. Hadley. They wrought together at Kew, and in May 1724, they finished a telescope 26 inches in focal length, and afterwards another of 8 feet, the largest that had yet been made. Encouraged by their success, Mr. Hawksbee made one of 3 1-4 feet, which bore a magnifying power of 226 times, and showed the black list, as it is called, or the division in Saturn's ring; and

He was probably the member of the firm of Reeves and Cox, celebrated glass grinders of that day, who failed in executing the speculum of a six feet Gregorian reflector, which James Gregory had employed him to make for him.-BREWSTER's Life of Newton, p. 28.

Newton's time was too valuable to be Mr. Hadley executed another telescope of the spent in mechanical labor, and he therefore never resumed the construction of re-in 1726 he completed a Gregorian one. Newtonian form, of the same focal length, and flecting telescopes.* The Royal Society, This instrument was elegantly fitted up by

H employed a London optician to grind a Mr. Molyneux, and presented to his Majesty glass speculum for a reflector, four feet long, but John V., King of Portugal.-SMITH's Optics, vol. the glass was bad, and the experiment failed. ii., p. 363.

other opticians now began to manufacture [of optics and mechanics he encountered reflecting telescopes of various sizes, for fewer difficulties than might have been exsale. pected, and he at length succeeded in comOne of the most distinguished makers pleting Newtonian telescopes of various of reflecting telescopes, was our country-sizes, from two feet to twenty feet in focal man, James Short, whose telescopes great-length, and Gregorian ones from eight ly surpassed those of all the English opti-inches to ten feet in focal length. At this cians. He began his career in 1732, and time he had not discovered the direct having found out a method of giving his method which he subsequently possessed of specula the true parabolic figure, he execut-giving to specula the figure of any of the ed one fifteen inches in focal length, which conic sections, and in order to secure a exhibited all the five satellites of Saturn, a good instrument, he finished a number of feat which Cassini could perform only with specula, and selected the best of them for a refractor seventeen feet long. Mr. Short his telescopes. With this view he underexecuted several reflecting telescopes, with went the enormous labor, which none but glass specula quicksilvered on the back, and those who have made such instruments can Colin Maclaurin informs us that they were appreciate, of casting, grinding, and polishexcellent instruments. After Short had ing two hundred specula of seven feet foestablished himself in London in 1742, he cus, one hundred and fijty of ten feet, and received £630 for a 12 foot reflector, which above eighty of twenty feet, besides several he executed for Lord Thomas Spencer, and of the Gregorian form, and a great number in 1752 he finished another for the King of on the same principle as Dr. Smith of CamSpain for £1200. bridge's reflecting microscope. The earliNotwithstanding the rapid progress est of these instruments was completed in which was thus made in the improvement 1774, and was a five feet Newtonian reflecof the reflecting telescope, and the undoubt-tor, with which he observed the ring of Saed excellence of many of the instruments turn, and the satellites of Jupiter. In orwhich had been executed, no discovery of der to make use of specula of so great a the slightest importance had yet been achieved by them. The last discovery in the heavens had been made in 1636, by Cassini, with the refracting telescopes of Campani, and nearly three quarters of a century had elapsed without any extension of our knowledge of the solar and sidereal When we recollect the fine discoveries systems. This long interval, however, was which were made by increasing the aperone of those breathing times which often tures and focal lengths of the refracting precede grand intellectual movements. telescope, we cannot fail to anticipate The power of the refracting telescope had analogous effects from the increased magnibeen strained to the utmost, and the reflec-tude which Dr. Herschel thus gave to the tors, vigorous and promising in their infancy, were about to attain a power and magnitude which no astronomer had ventured to anticipate. It was reserved for Sir William Herschel to accomplish this great task, and by telescopes of gigantic size to extend the boundaries of the solar system, and lay open the hitherto unexplored recesses of the sidereal world.

focal length, he was driven to the invention and construction of a great variety of stands, and to these labors we owe his seven feet Newtonian telescope stand, a piece of mechanism of great ingenuity, which he perfected in 1778.

apertures and focal lengths of his specula. When he directed these instruments to the heavens in 1776, almost every night which he devoted to observation presented him with some new and interesting phenomenon. His first observations, which appeared in the Philosophical Transactions, were made on the periodical star in the neck of the Whale, and on the Lunar Mountains; Having acquired a taste for astronomy, but interesting though these were, they and a general knowledge of the science sunk into insignificance when compared from the popular writings of Ferguson, this with his discovery on the 13th March 1781, eminent individual was anxious to see with of a New Planet, having its diameter four his own eyes, the wonders of the planetary and a half times larger than our own earth, system. Fortunately for science the acqui- or 35,112 English miles. At first he desition of a telescope sufficient for such a scribed it as a comet, but a more careful purpose was beyond his means, and he re-study of its motions proved it to be a planet solved on the bold attempt to construct one of our own system, which revolved round with his own hands. From his knowledge the sun in 83 1-2 years, in a path far beyond

the orbit of Saturn, and at the distance of a twenty feet Newtonian telescope, having 1,800,000,000 miles from the sun, which a speculum nearly nineteen inches in diame is twice as far as the planet Saturn. Eu- ter, he found that all the nebulæ and clusrope rung with this great discovery. Asters of stars which had been published by tronomers of all nations anticipated with Messier and Mechain, could be resolved delight the future labors of the discoverer; and the name of Herschel, destined to receive new laurels in a succeeding generation, became known in every part of the civilized world. To the new planet which he had discovered he gave the name of the Georgium Sidus, in honor of George III., who, with the true munificence of a king, -enabled Dr. Herschel to devote the rest of his life to the study of the heavens. He accordingly took up his residence at Datchet, in the neighborhood of Windsor, and entered upon a career of discovery unparalleled in the history of science.

into an infinite number of small stars; and in examining the portion of the Milky Way which passes tbrough Orion's hand and club, he looked with amazement at the "glorious multitude of stars, of all possible sizes, that presented themselves to his view," and he made the calculation that a belt 15° long and 2° broad, contained no fewer than 50,000 stars, capable of being distinctly counted. During these observations he discovered 466 new nebula or luminous clouds, composed of stars, and he was led to a theory of the Milky Way, one of the boldest and most remarkable, and Our limits will not permit us to give even a yet probable, conceptions which human general sketch of these important research- genius has ventured to form. He consides; but viewed as the rich harvest which ered our solar system, and all the stars was reaped by the introduction of large re- which we can see with the eye, as placed flecting telescopes, we must take a rapid within, and constituting a part of the neglance of the most prominent of his discov-bula of the Milky Way, a congeries of maeries. One of the most valuable properties ny millions of stars, so that the projection of large reflectors was the power which they of these stars must form a luminous track gave the observer of viewing the image formed by the large speculum, directly by the eye-glass, without using a small reflector. This method, called the Front view, was nearly equivalent to doubling the area of the speculum, as one half of the incident light is lost by reflection. Upon viewing the Georgium Sidus in this manner, Sir W. Herschel discovered on the 11th January 1787, the second and fourth of its satellites, and in 1790 and 1794, the first, third, fifth, and sixth, all of which revolved in a retrograde direction round their primary, in orbits very nearly in the same plane, and almost perpendicular to the plane of the ecliptic.

on the concavity of the sky; and by estimating or counting the number of stars in different directions, he was able to form a rude judgment of the probable form of the nebula, and of the probable position of the solar system within it.

In reason.

These views were still farther extended in a subsequent memoir, entitled Remarks on the Construction of the Heavens. He regarded the starry firmament as composed of twelve different classes of bodies. Insulated stars;-binary sidereal systems or double stars; more complex systems, or treble, quadruple, quintuple, or multiple stars;clustering stars, and the milky way;-clus ters of stars;-nebulæ ;-stars with burrs When we consider the many thousand or stellar nebulæ ;--milky nebulosity;stars which present themselves to the as-nebulous stars ;-planetary nebulæ ;-and tronomer's eye while applying a telescope to planetary nebula with centres. the heavens, and their almost perfect simi- ing upon these combinations of sidereal larity, differing from each other chiefly in matter, Dr. Herschel supposes that double their size and brightness, we can scarcely and multiple stars have a motion of rotation conceive it within the limits of human gen-round their common centre of gravity; that ius to do any thing more than count and the various nebulosities above mentioned name them, group them into constellations, and determine their relative places in the heavens. This, indeed, was all that had been done before Dr. Herschel's time; but no sooner did he discover the power of his own instruments than he undertook the Herculean task of gauging the heavens, and ascertaining their construction. With

are condensed by attraction, and converted into stars; that stars previously formed attract nebulous matter, and increase in size, and that neighboring stars slowly advance towards each other, and constitute globular clusters.

Theoretical as these views doubtless are, they are in entire harmony with the laws of

the material world, and some of them have been actually demonstrated by the subsequent discoveries of Sir W. Herschel and other astronomers. In more than fifty of the double stars, he found that in the space of a quarter of a century a change had taken place either in the distance of the stars, or in their angle of position, that is, in the angle which a line joining the stars forms with the direction of their daily motion, and that in some stars both their distance and their angle of position had changed. From a comparison of his earliest with his latest observations, he concluded that the smaller of the two stars revolved round the greater, in periods given in the following table:

Castor, d Serpentis, y Virginis, Leonis,

& Bootes,

Period of Revolution.

342 years.

375

708

1200

1681

reflector in 1781, but the speculum, which was three feet in diameter, having cracked in the act of annealing, and another of the same size having been lost in the fire from a failure in the furnace, his scheme was unexpectedly retarded. In ardent minds, however, disappointment is often a stimulus to higher achievements, and the double accident which we have mentioned suggested, no doubt, the idea of making a larger instrument. He accordingly intimated the plan of such a telescope to the King, through Sir Joseph Banks, that liberal and unwearied patron of science, and his Majesty, with that munificent spirit which he had previously displayed, instantly offered to defray the whole expense of it. Encouraged by this noble act of liberality, which has never been imitated by any other British sovereign, Sir W. Herschel, towards the close of the year 1783, began the Herculean task of constructing a reflecting telescope forty feet in length, and having a In the double star Hercules, the two speculum fully four feet in diameter. The stars had approached so near that five-metallic surface of the great speculum is eighths of the apparent diameter of the 49 inches in diameter, but upon the rim small star were actually eclipsed by the there is an offset one inch deep and threelarger one, so that the two together resem-fourths of an inch broad, which reduces the bled a single lengthened or wedge-formed polished or effective surface to 48 inches. In the double star Ursa Majoris, The thickness of the speculum, which is Sir William discovered an unusually rapid uniform in every part, is 3 inches, and its change of place, and it appears from the weight nearly 2118 pounds. The metal more recent observations of Struve, Sir" was composed of pure copper and pure John Herschel, and Sir James South, that tin, in the proportion of 430 lb. of copper its motion is very unequal, varying from to 2441 lb. of a higher speculum metal, about 5° to probably 200 or 30° per annum, whose proportions were 1496 copper and so that the rotation of the one star round 812 tin," -a quantity which Sir John Herthe other must be accomplished in about schel considers too low to resist tarnish. forty years! The composition used by Mudge was 32 The last great discovery made by Sir copper and 14 grain tin. Sir W. HerWilliam Herschel is the direction and mag-schel's, when reduced to this standard, was nitude of the proper motion of the fixed 32 copper, and 10.7 of tin. In his first atstars. This motion was discovered by Hal-tempt to cast the speculum, Sir William ley, and explained by Tobias Mayer, who used an inferior metal, which it is not easy to ascribed it to a motion of the whole solar identify, from his description of it. It was,. system. Sir W. Herschel ascertained that | however, a failure, and so was his second our solar system is advancing towards the attempt, with probably a higher alloy. In constellation Hercules, or, more accurate- casting the third, which we have just dely, to a point in space whose right ascension scribed, he met with entire success. is 245° 52', 30", and north polar dis-had the pleasure of seeing this speculum tance 40° 22′, and that the quantity of this forty years ago, which was freely shown to motion is such, that to an astronomer us by its distinguished maker; and having placed in Sirius, our sun would appear to been familiar with the aspect of the compodescribe an arch of a little more than a sitions of Mudge and Edwards, we distinctsecond every year. ly recollect that the four feet speculum had the look of a good ordinary speculum, made of the usual proportion of copper and tin, but of course did not possess that peculiar color which this composition received

star.

Ambitious of gaining a still farther insight into the bosom of space, Sir W. Herschel resolved to attempt the construction of larger telescopes. He began a 30 feet aërial

We

from the addition of arsenic and silver. The bright, as rendered it impossible to mistake speculum, when not in use, was preserved or not to see them." In less than a month, from damp by a tin cover, which fitted upon a Sir William discovered, with the same inrim of close grained cloth, cemented on the strument, the seventh satellite of Saturn,— circumference of the speculum. The tube of " an object," says Sir John Herschel," of a the telescope was 39 feet 4 inches long, and far higher order of difficulty." Though disits width 4 feet 10 inches. It was made of covered, however, by this noble instrument, iron, and was 3000 lbs. lighter than if it had both these satellites, which are nearer the been made of wood. The observer was seat-planet than the five old ones, and revolve ed in a suspended moveable seat at the mouth of the tube, and viewed the image of the object with a magnifying lens or eye-piece. The focus of the speculum, or the place of the image, was within 4 inches of the lower side of the mouth of the tube, and came forward into the air, so that there was space for the part of the head above the eye, to prevent it from intercepting many of the rays that go from the object to the mirror. The eye-piece moved in a tube carried by a slider directed to the centre of the speculum, and fixed on an adjustible foundation at the mouth of the tube.*

round their primary in 23 and 323 hours, were afterwards distinctly recognized by Sir William Herschel with the twenty feet reflector. Both the sixth and seventh have been seen by Sir James South with his great Achromatic of thirteen inches aperture, and M. Lamont of Munich has seen the sixth, with an Achromatic of eleven inches aperture.

As a maker of large reflecting telescopes, Sir W. Herschel was followed by Mr. John Ramage, a merchant in Aberdeen, who, so early as 1806, had succeeded in making reflectors with specula six inches in diameThis magnificent structure, which used ter. In 1810 he constructed an instrument to be an object of wonder to all travellers whose focal length was eight feet, and the who passed Slough, was completed on the diameter of its mirror 9 inches. In 1817 27th August, 1789; and the very first mo- he executed a still larger one of 20 feet foment it was directed to the heavens, a new cal length, and with a speculum of 13 inbody was added to the solar system. This ches in diameter, now in the possession of discovery was recorded in the following Thomas Gordon, Esquire, of Buthlaw, in memorable words :-"In hopes of great Aberdeenshire. Since that time, he comsuccess with my 40 feet speculum, I defer- pleted three telescopes, each 25 feet in fored the attack upon Saturn till that should cal length, and with mirrors 15 inches in be finished; and having taken an early opportunity of directing it upon Saturn, the very first moment that I saw the planet I was presented with a view of six of its satellites, in such a situation, and so

diameter. One of them was sold to Captain Ross, R. N., the celebrated Arctic navigator, and another has been erected at the Royal Observatory, Greenwich.* This instrument, which was for some time in our possession, was an excellent telescope, and * In a correspondence which the author of this showed the double stars with great distinctarticle had with Sir William Herschel between ness. Mr. Ramage's greatest effort was 1802 and 1896-a correspondence marked with made in 1823, when he cast and polished that kindness and condescension which a great a speculum 21 inches in diameter, and 54 mind never fails to show to its inferiors in age feet in focal length. It was not erected on and knowledge-he mentioned his having composed a work on the subject of casting, grinding, a stand at the end of 1825, and we believe and polishing "mirrors for telescopes of all sizes, it has been purchased by Professor Nichol, in which the method of giving them not only the for the Observatory of Glasgow. parabolic form, but any other of the conic sections that may be required, is explained with perfect clearness, and supported by several thousands of facts." Sir William mentioned also, that Sir Joseph Banks, the President of the Royal Society, was acquainted with his intention of giving this work to the public, and that he was, in a few days, (Jan. 1805,) going to London to consult him on the subject. We regret much that other, and doubtless more important pursuits, have interfered with the publication of a work which could not fail to have possessed the highest interest, and to have contributed to the perfection of the reflecting telescope, and to the advancement of astronomy.

Notwithstanding Mr. Ramage's success in producing good instruments, yet no discovery whatever was made by any of them, and we must therefore consider the reflecting telescope as having reached its climax in the hands of Sir W. Herschel. It seemed in vain to aim at greater results without royal or national support, and still more

*This instrument is described, and a drawing of it, as erected upon its stand, given in the Transactions of the Astronomical Society, vol. ii., P. 413.