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TELESCOPE
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TELESCOPE , an See also:optical See also:instrument employed to view distant See also:objects. The See also:term " photographic telescope " has been applied to See also:instruments employed to See also:record the See also:appearance of See also:celestial objects by See also:photography. The word was coined by Demiscianus, a See also:Greek See also:scholar, at the See also:request of Federigo Cesi, founder of the Accademia dei Lincei, from the Greek riP e, far, and aKo1rwv, to see. It was used by Galileo as See also:early as 1612, and came into See also:English use much later, when it supplanted See also:trunk and See also:cylinder, the terms hitherto used to denote the telescope.
See also:HISTORY
The See also:credit of the See also:discovery of the telescope has been a fruitful subject of discussion. Thus, because See also:Democritus announced that the Milky Way is composed of vast multitudes of stars, it has been maintained that he could only have been led to See also:form such an See also:opinion from actual examination of the heavens with a telescope. Other passages from the Greek and Latin authors have similarly been cited to prove that the telescope was known to the ancients. But, as has been remarked by Dr See also:Robert See also: 377 of See also:Jebb's edition, 1733, translated as follows:
" Greater things than these may be performed by refracted See also:vision. For it is easy to understand by the canons above mentioned that the greatest objects may appear exceedingly small, and the contrary, also that the most remote objects may appear just at See also:hand, and the converse; for we can give such figures to transparent bodies, and dispose them in such See also:order with respect to the See also:eye and the objects, that the rays shall be refracted and See also:bent towards any See also:place we please, so that we shall see the See also:object near at hand or at any distance under any See also:angle we please. And thus from an incredible distance we may read the smallest letters, and may number the smallest particles of dust and See also:sand, by See also:reason of the greatness of the angle under which we see them. . . . Thus also the See also:sun, See also:moon and stars may be made to descend hither in appearance, and to be visible over the heads of our enemies, and many things of the like sort, which persons unacquainted with such things would refuse to believe."
Molyneux also cites from Bacon's Epistola ad Parisiensem, " Of the Secrets of See also:Art and Nature," See also:chap. 5:
" Glasses or diaphanous bodies may be so formed that the most remote objects may appear just at hand, and the contrary, so that
we may read the smallest letters at an incredible distance, and may number things, though never so small, and may make the stars also appear as near as we please."
These passages certainly prove that Bacon had very nearly, if not perfectly, arrived at theoretical See also:proof of the possibility of constructing a telescope and a See also:microscope; but his writings give no See also:account of the trial of an actual telescope, nor any detailed results of the application of a telescope to an examination of the heavens. It has been pointed out by Dr Robert See also: See also: In the library of the university of See also:Leyden, amongst the See also:MSS. of See also:Huygens there is an original copy of a document (dated 17th See also:October 1608) addressed to the stateseneral by See also:Jacob Andrianzoon (the same individual who is called Tames Metius by Descartes), petitioning for the exclusive right of selling an instrument of his invention by which distant objects appear larger and more distinct. He states that he had discovered the instrument by accident when engaged in making experiments, and had so far perfected it that distant objects were made as visible and distinct by his instrument as could be done with the one which had been lately offered to the states by a See also:citizen and spectacle-maker of Middelburg. Among the acts of the states-See also:general pre-served in the See also:government archives at The Hague, Van Swinden found that on 2nd October 1608 the See also:assembly of the states took into See also:consideration the See also:petition of Hans Lippershey, spectacle-maker, a native of See also:Wesel and an inhabitant of Middelburg, inventor of an instrument for seeing at a distance. On 4th October a See also:committee was appointed to test the instrument, and on the 6th of the same See also:month the assembly agreed to give Lippershey 90o florins for his instrument. Further, on the 15th See also:December of the same See also:year they examined an instrument invented by Lippershey at their request to see with both eyes, and gave him orders to execute two similar instruments at goo florins each; but, as many other persons had knowledge of this new invention to see at a distance, they did not deem it expedient to grant him an exclusive See also:privilege to sell such instruments. The See also:dates of these documents dispose effectually of Borel's statement that Lippershey borrowed the ideas of Jansen in i61o. They also prove that, whilst Metius was in possession of a telescope, with which he may have experimented, about the See also:time when Lippershey presented his application for patent rights, yet he makes no pretension that Lippershey borrowed the invention from him. The conclusion is that Lippershey was the first See also:person who independently invented the telescope, and at the same time made the instrument known to the See also:world. The See also:common See also:story is that Lippershey, happening one day, whilst holding a spectacle-See also:lens in either hand, to See also:direct them towards the See also:steeple of a neighbouring See also: The inverting telescope, composed of two convex lenses, was a later invention; still it is not impossible that the original experiment was made with two convex lenses. Telescopes seem to have been made in Holland in consider-able See also:numbers soon after the date of their invention, and rapidly found their way over See also:Europe. Sirturus, in his De Telescopic (1618), states that " a Frenchman proceeded to See also:Milan in the month of May 1609 and offered a telescope for See also:sale to See also:Count di Fuentes "; and Lorenzi Pigorna writes,4 under date 31st See also:August 1609, that " Galileo had been appointed lecturer at See also:Padua for See also:life on account of a perspective like the one which was sent from See also:Flanders to See also:Cardinal See also:Borghese." See also:Simon See also:Marius, the See also:German astronomer, appears to have made astronomical observations in 1609 with a telescope which he procured from Holland, and See also:Professor S. P. See also:Rigaud of Oxford found from the MSS. of See also:Harriot, the mathematician, that he had been making astronomical observations with a Dutch telescope as early as See also:July 1609. Galileo, in his Nuncius Sidereus, states that, happening to be in See also:Venice about the month of May 16o9, he heard that a Belgian had invented a perspective instrument by means of which distant objects appeared nearer and larger, and that he discovered its construction by considering the effects of See also:refraction. In his Saggiatore Galileo states that he solved the problem of the construction of a telescope the first See also:night after his return to Padua from Venice, and made his first telescope next day by fitting a convex lens in one extremity of a leaden tube and a concave lens in the other one. A few days afterwards, having succeeded in making a better telescope than s See Dr G. Moll of See also:Utrecht, in Journ. See also:Roy. Inst., vol. i., 1831. 4 See also:Lease d' Uomini Illustri, p. 112 (Venice, 1744). the first, he took it to Venice, where he communicated the details of his invention to the public, and presented the instrument itself to the See also:doge Leonardo Donato, sitting in full See also:council. The See also:senate, in return, settled him for life in his lectureship at Padua and doubled his See also:salary, which was previously 500 florins and which then became See also:treble that which any of his predecessors had enjoyed. Galileo may thus claim to have invented the telescope independently, but not till he had heard that others had done so. In fact the time was ripe; and, as often happens in similar circumstances, only a hint was necessary to complete the latent See also:chain of thought. Galileo devoted all his time to improving and perfecting the telescope. Knowing the theory of his instrument, and possessed of much practical skill, coupled with unwearied See also:patience, he conquered the difficulties of grinding and polishing the lenses, and soon succeeded in producing telescopes of greatly increased See also:power. His first telescope magnified three diameters; but he soon made instruments which magnified eight diameters, and finally one that magnified thirty-three diameters.' With this last instrument he discovered in 1610 the satellites of See also:Jupiter, and soon afterwards the spots on the sun, the phases of See also:Venus, and the hills and valleys on the moon. He demonstrated the rotation of the satellites of Jupiter round the See also:planet, and gave rough predictions of their configurations, proved the rotation of the sun on its See also:axis, established the general truth of the Copernican system as compared with that of See also:Ptolemy, and fairly routed the fanciful dogmas of the philosophers. These brilliant achievements, together with the immense improvement of the instrument under the hands of Galileo, overshadowed in a See also:great degree the credit due to the original discoverer, and led to the universal See also:adoption of the name of the Galilean telescope for the form of the instrument invented by Lippershey. Kepler first explained the theory and some of the practical advantages of a telescope constructed of two convex lenses in his Catoptrics (1611). The first person who actually constructed a telescope of this form was the Jesuit Christoph Scheiner, who gives a description of it in his See also:Rosa Ursina (1630). William See also:Gascoigne was the first who practically appreciated the See also:chief advantages of the form of telescope suggested by Kepler, viz., the visibility of the image of a distant object simultaneously with that of a small material object placed in the common See also:focus of the two lenses. This led to his invention of the See also:micrometer and his application of telescopic See also:sights to astronomical instruments of precision (see MICROMETER). But it was not till about the See also:middle of the 17th See also:century that Kepler's telescope came into general use, and then, not so much because of the advantages pointed out by Gascoigne, but because its See also: James See also:Bradley, on 27th December 1722, actually measured the See also:diameter of Venus with a telescope whose object-See also:glass had a focal length of 212; ft. In these very See also:long telescopes This last power could not be exceeded with See also:advantage in this form of telescope till after the invention of the achromatic object-glass.no tube was employed, and they were consequently termed aerial telescopes. Huygens contrived some ingenious arrangements for directing such telescopes towards any object visible in the heavens—the focal See also:adjustment and centring of the eye-piece being preserved by a braced See also:rod connecting the object-glass and eye-piece. Other contrivances for the same purpose are described by Philippe de la Hire (Mom. de l'Acad., 1715) and by Nicolaus Hartsoeker (Miscel. Berol., 1710, vol. i. p. 261). Telescopes of such great length were naturally difficult to use, and must have taxed to the utmost the skill and patience of the observers. One cannot but pay a passing See also:tribute of admiration to the men who, with such troublesome tools, achieved such results. Reflecting Telescopes: Until Newton's discovery of the different refrangibility of See also:light of different See also:colours, it was generally supposed that object-glasses of telescopes were subject to no other errors than those which arose from the spherical figure of their surfaces, and the efforts of opticians were chiefly directed to the construction of lenses of other forms of curvature. James See also:Gregory, in his Optica Promota (1663), discusses the forms of images and objects produced by lenses and mirrors, and shows that when the surfaces of the lenses or mirrors are portions of See also:spheres the images are curves concave towards the See also:objective, but if the curves of the surfaces are conic sections the spherical See also:aberration is corrected. He was well aware of the failures of all attempts to perfect telescopes by employing lenses of various forms of curvature, and accordingly proposed the form of reflecting telescope which bears his name. But Gregory, according to his own See also:confession, had no practical skill; he could find no optician capable of realizing his ideas, and after some fruitless attempts was obliged to abandon all See also:hope of bringing his telescope into practical use. Newton was the first to construct a reflecting telescope. When in 1666 he made his discovery of the different refrangibility of light of different colours, he soon perceived that the faults of the refracting telescope were due much more to this cause than to the spherical figure of the lenses. He over-hastily concluded from some rough experiments (See also:Optics, bk. i. pt. ii. prop. 3) " that all refracting substances diverged the prismatic colours in a See also:constant proportion to their mean refraction"; and he See also:drew the natural conclusion " that refraction could not be produced without See also:colour," and therefore " that no improvement could he expected from the refracting telescope " (Treatise on' Optics, p. 112). But, having ascertained by experiment that for all colours of light the angle of incidence is equal to the angle of reflexion, he turned his See also:attention to the construction of reflecting telescopes. After much experiment he selected an alloy of See also:tin and See also:copper as the most suitable material for his specula, and he devised means for grinding and polishing them. He did not See also:attempt the formation of a parabolic figure on account of the probable See also:mechanical difficulties, and he had besides satisfied himself that the See also:chromatic and not the spherical aberration formed the chief faults of previous telescopes. Newton's first telescope so far realized his expectations that he could see with its aid the satellites of Jupiter and the horns of Venus. Encouraged by this success, he made a second telescope of 63-in. focal length, with a magnifying power of 38 diameters, which he presented to the Royal Society of See also:London in December 1671. A third form of reflecting telescope was devised in 1672 by Cassegrain (See also:Journal See also:des S4avans, 1672). No further practical advance appears to have been made in the See also:design or construction of the instrument till the year 1723, when See also: The instrument was examined by See also:Pound and Bradley, the former of whom reported upon it in Phil. Trans., 1723, No. 378, p. 382. After remarking that Newton's telescope " had lain neglected these fifty years," they stated that Hadley had sufficiently shown " that this See also:noble invention does not consist in See also:bare theory." They compared its performance with that of the object-glass of 123-ft. focal length presented to the Royal Society by Huygens, and found that Hadley's reflector " will See also:bear such a See also:charge as to make it magnify the object as many times as the latter with its due charge, and that it represents objects as distinct, though not altogether so clear and See also:bright... . Notwithstanding this difference in the brightness of the objects, we were able with this reflecting telescope to see whatever we have hitherto discovered with the Huygenian, particularly the transits of Jupiter's satellites and their shadows over his disk, the See also:black See also:list in Saturn's ring, and the edge of his See also:shadow See also:cast on his ring. We have also seen with it several times the five satellites of Saturn, in viewing of which this telescope had the advantage of the Huygenian at the time when we compared them; for, being in summer, and the Huygenian telescope being managed without a tube, the See also:twilight prevented us from seeing in this some of these small objects which at the same time we could discern with the reflecting telescope." Bradley and Molyneux, having been instructed by Hadley in his methods of polishing specula, succeeded in producing some telescopes of considerable power, one of which had a focal length of 8 ft.; and, Molyneux having communicated these methods to See also:Scarlet and See also:Hearn, two London opticians, the manufacture of telescopes as a matter of business was commenced by them (Smith's Oplicks, bk. iii. ch. 1). But it was reserved for James See also:Short of See also:Edinburgh to give practical effect to Gregory's original See also:idea. See also:Born at Edinburgh in 1710 and originally educated for the church, Short attracted the attention of See also:Maclaurin, professor of See also:mathematics at the university, who permitted him about 1732 to make use of his rooms in the See also:college buildings for experiments in the construction of telescopes. In Short's first telescopes the specula were of glass, as suggested by Gregory, but he afterwards used metallic specula only, and succeeded in giving to them true parabolic and elliptic figures. Short then adopted telescope-making as his profession, which he practised first in Edinburgh and after-wards in London. All Short's telescopes were of the Gregorian form, and some of them retain even to the See also:present day their original high See also:polish and See also:sharp See also:definition. Short died in London in 1768, having realized a considerable See also:fortune by the exercise of his profession.
Achromatic Telescope.—The See also:historical sequence of events now brings us to the discovery of the achromatic telescope. The first person who succeeded in making achromatic refracting telescopes seems to have been See also:Chester See also:Moor See also: Prosser of the Patent See also:Office to the society. On the same occasion A. C. Ranyard made the following interesting statement respecting Hall:—_ " Some years ago very little was known about Moor Hall. It was known that, about seven years after the patent for making achromatic object-glasses was granted to Dollond, his claim to the invention was disputed by other instrument-makers, amongst them by a Mr Champness, an instrument-maker of Cornhill, who began to infringe the patent, alleging that John Dollond was not the real inventor, and that such telescopes had been made twenty-five years before the granting of his patent by Mr Moor Hall. John Dollond, to whom the See also:Copley See also:medal of the Royal Society had beenthe first inventor of the achromatic telescope; but it was ruled by See also:Lord See also:Mansfield that " it was not the person who locked his invention in his scrutoire that ought to profit for such invention, but he who brought it forth for the benefit of mankind."3 In 1747 Leonhard See also:Euler communicated to the See also:Berlin See also:Academy of Sciences a memoir in which he endeavoured to prove the possibility of correcting both the chromatic and the spherical aberration of an object-glass. Like Gregory and Hall, he argued that, since the various humours of the human eye were so combined as to produce a perfect image, it should be possible by suitable combinations of lenses of different refracting media to construct a perfect object-glass. Adopting a hypothetical See also:law of the See also:dispersion of differently coloured rays of light, he proved analytically the possibility of constructing an achromatic object-glass composed of lenses of glass and See also:water. But all his efforts to produce an actual object-glass of this construction were fruitless—a failure which he attributed solely to the difficulty of procuring lenses worked precisely to the requisite curves (Mem. Acad. Berlin, 1753). Dollond admitted the accuracy of Euler's See also:analysis, but disputed his See also:hypothesis on the grounds that it was purely a theoretical See also:assumption, that the theory was opposed to the results of Newton's experiments on the refrangibility of light, and that it was impossible to determine a physical law from See also:analytical reasoning alone (Phil. Trans., 1753, p. 289). In 1754 Euler communicated to the Berlin Academy a further memoir, in which, starting from the hypothesis that light consists of vibrations excited in an elastic fluid by luminous bodies, and that the difference of colour of light is due to the greater or less frequency of these vibrations in a given time, he deduced his previous results. He did not doubt the accuracy of Newton's experiments quoted by Dollond, because he asserted that the difference between the law deduced by Newton and that which he assumed would not be rendered sensible by such an experiment.' Dollond did not reply to this memoir, but soon after-wards he received an abstract of a memoir by See also:Samuel Klingenstierna, the See also:Swedish mathematician and astronomer, which led him to doubt the accuracy of the results deduced by Newton on the dispersion of refracted light. Klingenstierna showed from purely geometrical considerations, fully appreciated by Dollond, that the results of Newton's experiments could not be brought into See also:harmony with other universally accepted facts of refraction. Like a practical man, Dollond at once put his doubts to the test of experiment, confirmed the conclusions of Klingenstierna, discovered " a difference far beyond his hopes in the refractive qualities of different kinds of glass with respect to their divergency of colours," and was thus rapidly led to the construction of object-glasses in which first the chromatic and afterwards the spherical aberration were corrected (Phil. Trans., 1758, p. 733). We have thus followed somewhat minutely the history of the See also:gradual See also:process by which Dollond arrived independently at his invention of the refracting telescope, because it has been asserted that he borrowed the idea from others. See also:Montucla, given for his invention, was the dead, and his son brought an See also:action for infringing the patent against Champness. There is no See also:report of the See also:case, but the facts are referred to in the reports of subsequent cases. It appears that workmen who had been employed by Mr Moor Hall were examined, and proved that they had made achromatic object-glasses as early as 1733. Dollond's patent was not set aside, though the evidence with regard to the See also:prior manufacture was accepted by Lord Mansfield, who tried the case, as having been satisfactorily proved . . . Mr Hall was a bencher of the Inner See also:Temple, and was alive at the time of the action. He was a man of some See also:property, and is spoken of on hits tombstone as an excellent lawyer and mathematician. He was not a See also:fellow of the Royal Society, but must certainly have known of the See also:gift of the Copley medal to Dollond. It is very curious the conflicting evidence we have to reconcile, but I think the See also:balance of evidence is in favour of there having been a prior invention of achromatic object-glasses before the date of Dollond's patent" (Astron. See also:Register, May 1886; see also the See also:Observatory for same date). 2 Gentleman's See also:Magazine, 1790, See also:part ii. p, 89o. ' For a See also:good account of this controversy, see Dr H. Servus, Geschichte des Fernrohrs, P. 77 seq. (Berlin, 1886). in his Idistoire des Mathbmatiques (pp. 448-440, gives the following footnote, communicated to him by See also:Lalande:--- " Cc fut Chestermonhall " (an obvious misprint for Chester Moor Hall) qui, vers 1750, eut l'idec des lunettes achromatiques. II s'adressoit a See also:Ayscough' qui faisoit travaillir See also:Bass. Dollond ayant cu besoin de Bass pour un verre que demandoit le duc d'Yorck, Bass lui See also:fit voir du See also:crown-glass et du See also:flint-glass. Hall donna une See also:lunette a Ayscough, qui la montra a plusieurs personnes; it en donna la construction a See also:Bird, qui n'en tint pas compte. Dollond en profits. Dans le proves qu'il y eut entre Dollond et Watkin, au bane du roi, cela fut prouee; mais Dollond gagna, parce qu'il etoit le premier qui eIIt fait connoitre See also:les lunettes achromatiques." It is clearly established that Hall was the first inventor of the achromatic telescope; but Dollond did not See also:borrow the invention from Hall without See also:acknowledgment in the manner suggested by Lalande. His discovery was beyond question an independent one. The whole history of his researches proves how fully he was aware of the conditions necessary for the attainment of See also:achromatism in refracting telescopes, and he may be well excused if he so long placed implicit reliance on the accuracy of experiments made by so illustrious a philosopher as Newton. His writings sufficiently show that but for this confidence he would have arrived sooner at a discovery for which his mind was fully prepared. It is, besides, impossible to read Dollond's memoir (Phil. Trans., 1758, p. 733) without being impressed with the fact that it is a truthful account, not only of the successive steps by which he independently arrived at his discovery, but also of the logical processes by which these steps were successively suggested to his mind. The triple object-glass, consisting of a combination of two convex lenses of crown glass with a concave flint lens between them, was introduced in 1765 by Peter, son of John Dollond, and many excellent telescopes of this See also:kind were made by him. The limits of this See also:article do not permit a further detailed historical statement of the various steps by which the See also:powers of the telescope were See also:developed. Indeed, in its practical form the principle of the instrument has remained unchanged from the time of the Dollonds to the present day; and the history of its development may be summed up as consisting not in new optical discoveries but in utilizing new appliances for figuring and polishing, improved material for specula and lenses, more refined means of testing, and more perfect and convenient methods of mounting. About the year 1774 William See also:Herschel, then a teacher of See also:music in See also:Bath, began to occupy his leisure See also:hours with the construction of specula, and finally devoted himself entirely to their construction and use. In 1778 he had selected the chef-d'oeuvre of some 400 specula which he made for the celebrated instrument of 7-ft. focal length with which his early brilliant astronomical discoveries were made. In 1783 he completed his reflector of 184 in. aperture and 20-ft. focus, and in 1789 his great reflector of 4-ft. aperture and 4o-ft. focal length. The fame of these instruments was rapidly spread by the brilliant discoveries which their maker's See also:genius and per-severance accomplished by their aid. The reflecting telescope became the only available See also:tool of the astronomer when great light grasp was requisite, as the difficulty of procuring disks of glass (especially of flint glass) of suitable purity and homogeneity limited the dimensions of the achromatic telescope. It was in vain that the See also:French Academy of Sciences offered prizes for perfect disks of optical flint glass. Some of the best chemists and most enterprising glass-manufacturers exerted their utmost efforts without succeeding in producing perfect disks of more than 3 z in. in diameter. All the large disks were crossed by striae, or were otherwise deficient in the necessary homogeneity and purity. The subsequent history of the development of the art of manufacturing glass disks for telescopic objectives will be found in the article GLASS: § Optical. Additional information and CommentsThere are no comments yet for this article.
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