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fes at a high price, and took every poffible method to keep his art a fecret. His laboratory was inacceffible till after his death; when it was pur-chafed by Pope Benedict XIV. who made a prefent of it to the Inftitute of that city; and by the account which M. Fougeroux has given of what he could discover from it, we learn, that (except a machine, which M. Campani conftructed, to work the bafons on which he ground his glasses) the goodness of his lenfes depended upon the clearnefs of his glafs, his Venetian tripoli, the paper with which he polished his glaffes, and his great fkill and addrefs as a workman. He made few lenfes of a very great focal diftance; and having the misfortune to break one of 141 feet in two pieces, he took incredible pains to join the two parts together, which he did at length effectually, fo that it was ufed as if it had been entire.

179. SIR PAUL NEILLE, Dr Hooke fays, made teleicopes of 36 feet, pretty good, and one of so. Afterwards Mr REIVE, and then Mr Cox, who were the most celebrated in England as grinders of optic glaffes, made fome good ones of 50 and 60 feet focal distance, and Mr Cox made one of 100. BORELLI alfo, in France, made object glaffes of a great focal length, one of which he prefented to the Royal Society. With refpect to the focal length of telescopes, thefe and all others were far exceeded by M. Auzour, who made one object-glafs of too feet focus; but he was rever able to make any ufe of it. HARTSOCKER is even faid to have made fome of a ftill greater focal length; but this ingenious mechanic, finding it impoffible to make use of object-glaffes the focal diftance of which was much lefs than this, when they were inclosed in a tube, contrived a method of ufing them without a tube, by fixing them at the top of a tree, a high wall, or the roof of a house."

180. Mr HUYGENS alfo made confiderable improvements in the method of ufing an object glass without a tube. He placed it at the top of a very long pole, having previously inclofed it in a fhort tube, which was made to turn in all directions, by a ball and focket. The axis of this tube, he could command with a fine filken ftring, fo as to bring it into a line with the axis of another fhort tube, which he held in his hand, and which contained the eye-glafs. He thus could ufe object glaffes of the greateft magnifying power, at whatever altitude his object was, and even in the zenith, provided his pole was as long as his tele fcope; and, to adapt it to the view of objects of different altitudes, he had a contrivance, by which he could raise or deprefs a ftage that fupported his object-glafs at pleasure.

18r. M. DE LA HIRE made fome improvement in this method of managing the object-glafs, fixing it in the centre of a board, and not in a tube; but as it is not probable that this method will ever be made ufe of, fince the difcovery of both reflecting and achromatic telefcopes, which are now brought to great perfection, we need not deferibe this apparatus minutely; but only give a figure of Mr Huygens's pole, which, with a very short explanation, will be fufficient. In fig. 1. Pl. CCLI. a reprefents a pulley, by the help of which a ftage c, d, e, f, (that supports the object-glafs k, and the VOL. XVI. PART L

apparatus belonging to it), may be raised higher or lower at pleasure, the whole being counterpoifed by the weight h, faftened to a string gi; n is a weight, by means of which the centre of gravity of the apparatus belonging to the object-glass is kept in the ball and focket, fo that it may be eafily managed by the ftringu, and its axis brought into a line with the eye-glafs at o. When it was very dark, M. Huygens was obliged to make his object-glafs vifible by a lantern y, fo conftructed as to throw the rays of light in a parallel direction up to it.

182. M. AUZOUT, in a paper delivered to the Royal Society, obferved, that the apertures which the object glaffes of refracting telescopes can bear with diftinétnefs, are in about a fub-duplicate proportion to their lengths; and upon this fuppofi. tion he drew up a table of the apertures proper for object-glaffes of a great variety of focal lengths, from 4 inches to 4c0 feet. Upon this occafion, Dr. Hooke observed, that the fame glass will bear a greater or lefs aperture, according to the lefs or greater light of the object. If, for inftance, he was viewing the fun, or Venus, or any of the fixed stars, he used fmaller apertures: but if he wanted to view the moon by day-light; or Saturn, Jupiter, or Mars, by night, he ufed a larger aperture.

183. But the merit of all thofe improvements was in a manner cancelled by the discovery of the much more commodious REFLECTING TELESCOPE. For a refracting telescope, even of 1000 feet focus, fuppofing it poffible to be made ufe of, could not be made to magnify with diftinctnefs more than 1000 times; whereas a reflecting telescope, not exceeding 9 or 10 feet, will magnify 1200 times. "It must be acknowledged (fays Dr SMITH in his Complete Syftem of Optics), that Mr JAMES GREGORY of Aberdeen, was the firft inventor of the reflecting telescope." But, according to Dr PRINGLE, MERSENNUS was the man who entertained the first thoughts of a reflector. A telescope with fpecula he certainly propo, fed to the celebrated Defcartes many years before Gregory's invention, though indeed in a manner fo very unfatisfactory, that Defcartes, who had given particular attention to the improvement of the telescope, was fo far from approving the pro. pofal, that he endeavoured to convince Mertennus of its fallacy. See TELESCOPE.. Dr Smith had not feen two letters of Defcartes to Merfennus on that fubject.

184. GREGORY, a young man of an uncommon genius, was led to the invention, in feeking to correct two imperfections of the common tele. fcope: the firft was its too great length, which made it lefs manageable; the fecond, the incorrectnefs of the image. Mathematicians had demonftrated, that a pencil of rays could not be co!lected in a fingle point of a spherical lens; and alfo, that the image tranfmitted by fuch a lens would be in fome degree incurvated. Thefe inconveniences he believed would be obviated by fubstituting for the object-glafs a metallic fpeculum, of a parabolic figure, to receive the image, and to reflect it towards a fmall fpeculum of the fame metal: this again was to return the image to an eye-glafs placed behind the great fpeculum, which for that purpose was to be perforated in its centre. This Y Y

conftruction

conftruction he published in 1663, in his Optica Promota. But as Gregory was endowed with no mechanical dexterity, nor could find any workman capable of realizing his invention, after fome fruitless attempts in that way, he was obliged to give up the pursuit: and probably, had not some new discoveries been made in light and colours, a refracting telescope would never more have been thought of, confidering the difficulty of the execution, and the fmall advantages that could accrue from it, deducible from the principles of optics then known.

185. But NEWTON, whofe genius for experimental knowledge was equal to that for geometry, happily interpofed, and faved this noble invention from well nigh perifhing in its infant ftate. He likewife at an early period of life had applied him felf to the improvement of the telescope; but imagining that Gregory's fpecula were neither very neceffary, nor likely to be executed, he began with profecuting the views of Descartes, who aimed at making a more perfect image of an object, by grinding lenfes, not to the figure of a fphere, but to that of a conic fection. While he was thus employed, three years after Gregory's publication, he happened to examine the colours formed by a prifm, and having by that fimple inftrument difcovered the different refrangibility of the rays of light, he perceived that the errors of telefcopes, arifing from that cause alone, were fome hundred times greater than those occafioned by the fpherical figure of lenfes. This circumftance forced Newton to fall into Gregory's track, and to turn his thoughts to reflectors. The différent refrangibility of the rays of light (fays he, in a letter to Mr Oldenburg, fecretary to the Royal Society, dated Feb. 1672) made me take reflections into confideration; and finding them regular, fo that the angle of reflection of all forts of rays was equal to the angle of incidence, I under. ftood that, by their mediation, optic inftruments might be brought to any degree of perfection imaginable, providing a reflecting fubftance could be found which would polifh as finely as glafs, and reflect as much light as glafs tranfmits, and the art of communicating to it a parabolic figure be alfo obtained. Amidft thefe thoughts I was forced from Cambridge by the intervening plague, and it was more than two years before I proceed. ed further." If Newton, then, was not the firft inventor of the reflecting telescope, he was the main and effectual inventor. By his admirable genius, he fell upon this new property of light; and thereby found, that all lenses, of whatever figure, would be affected more or lefs with fuch prifmatic aberrations of the rays as would be an infuperable obftacle to the perfection of a dioptric telescope.

186. About the end of 1668, or beginning of 1669, Newton, not relying on any artificer for making his fpecula, fet about the work himfelf, and carly in 1672 completed two small reflecting telefcopes. In thefe he ground the great fpeculum into a fpherical concave, but found himfelf unable to accomplish the parabolic form. In the letter that accompanied one of thefe inftruments, which he prefented to the Society, he writes, "that though

he then defpaired of performing that work, (to wit, the parabolic figure of the great speculum) by geometrical rules, yet he doubted not but that the thing might in fome measure be accomplished by mechanical devices."

187. Not lefs did the difficulty appear to find a metallic fubftance that would be of a proper hardness, have the feweft pores, and receive the fmoothest polish: a difficulty which he deemed almost unfurmountable, when he confidered, that every irregularity in a reflecting furface would make the rays of light ftray 5 or 6 times more out of their due courfe, than the like irregularities in a refracting one. In another letter written foon after, he tells the secretary, "that he was very fenfible, that metal reflects lefs light than glafs tranfmits; but as he had found fome metallic fubftances to be more ftrongly reflective than others, to polish better, and to be freer from tarnishing than others, fo he hoped that there might in time be found out fome fubftances much freer from thefe inconveniences than any yet known." Newton therefore laboured till he found a compofition that anfwered in fome degree, and prefented a reflecting telescope, to the Royal Society; from whom he received fuch thanks as were due to fo curious and valuable a prefent. And HUYGENS, one of the greatest geniufes of the age, and a diftinguished improver of the refractor, no fooner was informed by Mr OLDENBURG of the discovery, than he wrote an answer, “that it was an admirable telefcope; and that Mr Newton had well confidered the advantage which a concave speculum had above convex glaffes in collecting the parallel rays, which, according to his own calculation, was very great; hence that Mr Newton could give a far greater aperture to that speculum, than to an object-glafs of the fame distance of facus, and confequently much more magnify in his way than by an ordinary telescope: Befides, that by the reflector he avoided an inconvenience infeparable from object-glaffes which was the obliquity of both their furfaces, which vitiated the refraction of the rays that pafs towards the fides of the glafs, and did more hurt than men were aware of: Again, that by the mere reflection of the metalline fpeculum there were not fo many rays loft as in glaffes, which reflected a confiderable quantity by each of their furfaces, and, befides, intercepted many of them by the obfcurity of their matter: That the main bufinefs would be, to find a matter for this fpeculum that would bear as good a polish as glass.”

188. M. HUYGENS was not fatisfied with thus expreffing to the Society his high approbation of Newton's invention; but drew up a favourable account of the new telescope, which he caused to be published in the Journal de Sçavans for 1672, by which it was foon known over Europe. But how excellent foever the contrivance was: how well foever fupported and announced to the public; yet whether it was that the artifts were deterred by the difficulty and labour of the work, or that the difcoveries even of a Newton were not to be exempted from the general fatality attending great and useful inventions, the making a flow and vexatious progress to the authors; the fact is,

that,

that, excepting an unfuccefsful attempt which the fociety made, by employing an artificer to imitate the Newtonian conftruction, but upon a larger feale, and a disguised Gregorian telescope, fet up by CASSEGRAIN abroad as a rival to New ton's, and that in theory only (for it was never put in execution by the author), no reflector was heard of for near half a century after. But a reflecting telescope was at laft produced to the world, of the Newtonian conftruction, by Dr HADLEY, which the author had the fatisfaction to find executed in fuch a manner, as left no room to fear that the invention would any longer continue in obfcurity.

189. This memorable event was owing to the genius, dexterity, and application, of Dr HADLEY, the inventor of the REFLECTING QUADRANT, another most valuable inftrument. The two telefcopes which Newton had made were but fix inches long, were held in the hand for viewing objects, and in power were compared to a fix-feet refractor; whereas Hadley's was above five feet long, was provided with a well-contrived appara tus for managing it, and equalled in performance the famous aerial telescope of Huygens of 123 feet in length. Excepting as to the manner of making the fpecula, we have, in the tranfactions of 1723, a complete defcription, with a figure, of this te. lescope, together with that of the machine for moving it; but, by a strange omiffion, Newton's name is not once mentioned in that paper, so that any perfon not acquainted with the hiftory of the invention, and reading that account only, might be apt to conclude that Hadley had been the fole inventor. The fame celebrated artift, after finishing two telescopes of the Newtonian conftruction, accomplished a 3d in the Gregorian way; but, it would feem, lefs fuccefsfully, by Dr Smith's declaring fo ftrongly in favour of the other. Dr Hadley inftructed Mr MOLYNEUX and the Rev. Dr BRADLEY; and when thefe gentlemen had made a fufficient proficiency in the art, being defirous that these telescopes fhould become more public, they liberally communicated to fome of the principal inftrument-makers of London the knowledge they had acquired from him. Such scholars foon advanced beyond their mafters, and completed reflectors by other and better methods than what had been taught them.

190. Mr JAMES SHORT, as early as 1734, had fignalifed himself at Edinburgh by his work of this kind. Mr MACLAURIN Wrote that year to Dr Jurin, "that Mr Short, who had begun with making glafs fpecula, was then applying himself to improve the metallic; and that, by taking care of the figure, he was enabled to give them larger apertures than others had done; and that upon the whole they furpaffed in perfection all that he had feen of other workmen." He added, "that Mr Short's telescopes were all of the Gregorian conftruction; and that he had much improved that excellent invention." This character of excellence Mr Short maintained to the laft; as he had been well grounded both in the geometrical and philofophical principles of optics, and upon the whole was a moft intelligent perfon in what ever related to his profeffion. It was fuppofed he had fallen upon a method of giving the parabolic

figure to his great fpeculum: a point of perfection which Hadley had never attempted, either in his Newtonian or Gregorian telescope. Mr Short, indeed, faid he had acquired that faculty, but never would tel! by what means he effected it; fo that the fecret of working that configuration died with that ingenious artitt. Mr MUDGE, however, hath lately realised the expectation of Sir Ifaac Newton, who above 100 years ago, prefaged that the public would one day poffefs a parabolic fpeculum, not accomplished by mathematical rules, but by mechanical devices.

191. This defideratum was not the only want fupplied by this gentleman: he taught us likewife a better compofition of metals for the fpecula, how to grind them better, and how to give them a finer polifh; and the polish, he remarks, was the most difficult and effential of the whole operation. "In a word, (fays Sir JOHN PRINGLE,) I am of opinion, there is no optician in this great city (which hath been fo long and fo juftly renowned for ingenious and dextrous makers of every kind of mathematical inftruments) fo partial to his own abilities as not to acknowledge, that, however fome parts of the mechanical procefs now difclofed might have been known before any indi viduals of the profeffion, yet that Mr MUDGE hath opened to them all fome new and important lights, and upon the whole hath greatly improved the art of making reflecting telescopes."

192. The late Rev. and ingenious JOHN EDWARDS devoted much of his time to the improvement of reflecting telescopes, and brought them to fuch perfection, that Dr MASKELYNE, the aftronomer royal, found telescopes conftructed by him to furpafs in brightnefs, and other effentials, thofe of the fame fize made by the best artifts in London. The chief excellence of his telescopes arifes from the compofition, which, from various trials on metals and femimetals, he discovered for the fpecula, and from the true parabolic figure, which, by long practice, he had found a method of giving them, preferable to any that was known before him. His directions for the compofition of fpecula, and for cafting, grinding, and polishing them, were publifhed, by order of the commiffioners of longitude, at the end of the Nautical Almanack for 1787: To which is alfo annexed his account of the caufe and cure of the tremors which particularly affect reflecting telescopes more than refracting ones, together with remarks on the said tremors by Dr Maskelyne. See TELESCOPE.

193. But in contructing reflecting telelcopes of extraordinary magnifying powers, Dr HERSCHEL has difplayed skill and ingenuity furpaffing all his predeceffors. He has made them from 7, 10, 20, to even 40 feet in length; and with the inftrument of thefe latter dimenfions he is now employed in making difcoveries in aftronomy. Of its conftruction, magnifying powers, and the cu rious collection of machinery, by which it is fupported and moved from one part of the heavens to another, accounts will be found under TELE SCOPE.

194. The greatest improvement in refr. & ng telefcopes hitherto made public is that of Mr DOLLOND, of which an account has been given above. (See 31-40.) But befides the obligation we are

Y y 2

under

a confiderable improvement on the former. Thus encouraged, he refolved to try if he could make fome farther enlargement of the field. by the addition of another glass, and by placing and proportioning the glaffes in fuch a manner as to correct the aberrations, without detriment, to the diftinctnefs. At last he obtained as large a field as is neceffary, even in the longest telescopes that can be made. Thefe telescopes with fix glaffes having been well received, and fome of them being gone into foreign parts, the author fixed the date of his invention, in a letter, addressed to Mr Short, which was read to the Royal Society, March 1. 1753.

under to him for correcting the aberration of the rays of light in the focus of object-glaffes, arifing from their different refrangibility, he made another confiderable improvement, viz. by correcting both this kind of aberration, and also that which arifes from the spherical form of lenfes, by an expedient of a very different nature; viz. increafing the number of eye-glaffes. If any perfon, fays he, would have the visual angle of telescope to contain 20° the extreme pencils of the field must be bent or refracted in an angle of 10°; which, if it be performed by one eye-glass, will caufe an aberration from the figure, in proportion to the cube of that angle; but if two glaffes are fo proportioned and fituated as that the refraction may 197. Various attempts were made about this be equally divided between them, they will each time, to fhorten and otherwise improve telescopes. of them produce a refraction equal to half the re- Among thefe we must mention that of Mr CALEB quired angle; and therefore, the aberration be- SMITH, who thought he had found it possible to ing in proportion to the cube of half the angle rectify the errors which arife from the different taken twice over, will be but a 4th part of that degrees of refrangibility, on the principle that the which is in proportion to the cube of the whole fines of refraction, or rays differently refrangible, angle; because twice the cube of 1 is but of are to one another in a given proportion, when the cube of 2; so the aberration from the figure, their fines of incidence are equal; and he prowhere two eye-glaffes are rightly proportioned, is pofed, for this purpose to make the fpeculums of but a 4th of what it muft unavoidably be, where glafs inftead of metal, the two furfaces having the whole is performed by a fingle eye-glafs. By different degrees of concavity. But his fcheme the fame reafoning, when the refraction is divided was never executed; nor is it probable, for reabetween three glasses, the aberration will be found fons which have been mentioned, that any advanto be but the ninth part of what would be pro- tage could be made of it. duced from a fingle glafs; because three times the cube of I is but one 9th of the cube of 3. Whence it appears, that by increafing the number of the eye-glaffes, the indiftinétnefs which is obferved near the borders of the field of a telescope may be very much diminished, though not entirely taken away.

195. The method of correcting the errors arif ing from the different refrangibility of light is of a different confideration. For, whereas the errors from the figure can only be diminished in a certain proportion according to the number of glaffes, in this they may be entirely corrected by the addition of only one glafs; as we find in the aftronomical telescope, that two eye-glaffes, rightly proportioned, will caufe the edges of objects to appear free from colours, quite to the border of the field. Alfo in the day telescope, where no more then two eye-glaffes are abfolute ly neceffary for erecting the object, we find, that by the addition of a 3d rightly fituated, the colours, which would otherwife make the image confufed, are entirely removed. This, however, is to be underflood with fome limitation; for if the glaffes exceed a certain length, the colours may be spread too wide to be admitted through the pupil of the eye; which is the caufe, that in long telescopes with 3 eye-glaffes, the field is al. ways greatly.contracted.

196. These confiderations firft fet Mr Dollond on contriving how to enlarge the field, by increafing the number of eye-glaffes without any hindrance to the distinctness or brightnefs of the image; and though others had been about the fame work before, yet, observing that fome fiveglafs telescopes then made would admit of farther improvement, he endeavoured to conftruct one with the fame number of glaffes in a better manner; which was allowed by the best judges to be

198. To Mr SHORT we are indebted for the excellent contrivance of an equatorial telescope, or portable obfervatory; whereby pretty accurate obfervations may be made with little trouble, by thofe who have no building adapted to the purpofe. This inftrument confifts of an ingenious piece of machinery, by the help of which a telefcope mounted upon it may be directed to any degree of right ascension or declination, fo that the place of any of the heavenly bodies being known, they may be found without any trouble, even in the day-time. Being made to turn parallel to the equator, any object is easily kept in view, or recovered, without moving the eye from its fituation. By this inftrument, Mr Short informs us, that most of the stars of the firft and second magnitude have been seen even at mid-day, and the fun shining bright; as alfo Mercury, Venus, and Jupiter. Saturn and Mars are not fo eafy to be fee, on account of the faintnefs of their light, except when the fun is but a few hours above the horizon. This particular effect depends upon the telescope excluding almost all the light, except what comes from the object itself, and which might otherwise efface the impreffion made by its weaker light upon the eye. For the fame reafon, ftars are vifible in the day-time from the bottom of a deep pit. Mr RAMSDEN has also invented a portable obfervatory or equatorial telescope. See ASTRONOMY, Index.

199. To enable us to fee the fixed ftars in the day-time, it is neceffary to exclude the extraneous light as much as poffible. For this reafon the greater magnifying power of any telescope is used, the more eafily a fixed ftar will be diftinguished in the day-time; the light of the ftar remaining the fame in all magnifying powers of the fame telefcope, but the ground upon which it is seen be coming darker by increafing the magnifying pow

er;

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