357. In July, 1610, Galileo being still at Padua, and getting an imperfect view of Saturn's ring, imagined that the planet consisted of three parts; and therefore, in the account which he gave of this discovery to his friends, he calls it planetam tergeminam. He also observed some spots on the face of the sun: but did not then publish his discovery; partly for fear of irritating the obstinate Aristotelians, and partly to make more exact observations on this remarkable phenomenon. He therefore contented himself with communicating his discovery to some of his friends at Padua and Venice, among whom was F. Paul. This delay, however, was the cause of this discovery being contested with him by the famous Scheiner, who likewise made the same observation in October 1611, and anticipated Galileo in the publication of it. About the end of August Galileo left Padua and went to Florence; and in November following he was satisfied, that from September Venus had been continually increasing in bulk, and that she changed her phases like the moon. About the end of March 1611 Galileo went to Rome, where he gratified the cardinals and the chief nobility with a view of the new wonders he had discovered in the heavens, and among others the solar spots. From these discoveries Galileo obtained the name of Lynceus, after one of the Argonauts, who was famous in antiquity for the acuteness of his sight; and the marquis of Monticelli instituted an academy, with the title of Lincei, and made him a member of it. Galileo enjoyed the use of his telescope twenty-nine years, continually enriching astronomy with his observations: but, by too close application during the nocturnal air, his eyes grew gradually weaker, till in 1639 he became totally blind; a calamity which, however, neither broke his spirits, nor interrupted the course of his studies. The first telescope that Galileo constructed magnified only three times; but soon after he made another which magnified eighteen times; and afterwards, with great trouble and expense, he constructed one that magnified thirty-three times and with this he discovered the satellites of Jupiter and the spots of the sun. 358. Although Galileo had great merit with respect to telescopes, yet he did not explain the rationale of the instrument. This important service to science was performed by John Kepler, whose name is famous in the annals of philosophy, especially by his discovery of the great law of motion respecting the heavenly bodies; which is that the squares of their periodical times are as the cubes of their distance from the body about which they revolve; a proposition which, however, was not demonstrated before Sir Isaac Newton. Kepler was astronomer to several emperors of Germany; he was the associate of the celebrated astronomer Tycho Brahe, and the master of Descartes. He made several discoveries relating to the nature of vision; and not only explained the rationale of the telescopes which he found in use, but also pointed out methods of constructing others of superior powers and more commodious use. Kepler first gave a clear explication of the effects of lenses in making the rays of a pencil of light converge or diverge. He showed that a plano-convex lens makes rays pa

rallel to its axis to meet at the distance of the diameter of the sphere of convexity; but that, if both sides of the lens be equally convex, the rays will have their focus at the distance of the radius of the circle corresponding to that degree of convexity. But he did not investigate any rule for the foci of lenses unequally convex. To Cavallieri we owe this investigation. He laid down this rule: as the sum of both the diameters is to one of them, so is the other to the distance of the focus. All these rules concerning convex lenses are applicable to those that are concave; with this difference, that the focus is on the contrary side of the glass.

359. The principal effects of telescopes depend upon these plain maxims, viz. That objects appear larger in proportion to the angles which they subtend at the eye; and the effect is the same whether the pencils of rays by which objects are visible to us come directly from the objects themselves, or from any place nearer to the eye, where they may have been united so as to form an image of the object; because they issue again from those points where there is no real substance, in certain directions, in the same manner as they did from the corresponding points in the objects themselves. All that is effected by a telescope is, first to make such an image of a distant object, by means of a lens or mirror; and then to assist the eye in viewing that image as near as possible; so that the angle, which it shall subtend at the eye, may be very large compared with the angle which the object itself would subtend. This is done by an eye-glass, which so refracts the pencils of rays as that they may afterwards be brought to their several foci by the natural humors of the eye. If, instead of an eye glass, an object may be looked at through a small hole in a thin plate or piece of paper, held close to the eye, it may be viewed very near to the eye, and, at the same distance, the apparent magnitude of the object will be the same in both cases, But, as very few rays can be admitted through a small hole, there is seldom light sufficient to view an object to advantage in this way.

360. Such was the telescope first discovered and used by astronomers; and it is of a much more difficult construction than some other kinds that have been invented since. The great inconvenience attending it is, that the field of view is exceedingly small. For, since the pencils of rays enter the eye very much diverging from one another, but few of them can be intercepted by the pupil; this inconvenience increases with the magnifying power of the telescope, so that philosophers at this day cannot help wondering that it was possible, with such an instrument, for Galileo and others to have made the discoveries they did. It must have required incredible patience and address. No other telescope, however, than this, was so much as thought of for many years after the discovery. Descartes, who wrote thirty years after, mentions no others as actually constructed. To Kepler we are indebted for the invention of the astronomical telescope, the best adapted for viewing the heavenly bodies. The rationale of this instrument is explained, and the advantages of it are clearly pointed out, by this philosopher, in his Catoptrics; although he ne

Mr. Cox made one of 100. Borelli also, in France, made object-glasses of a great focal length, one of which he presented to the Royal Society. With respect to the focal length of telescopes, these and all others were far exceeded by M. Auzout, who made one object-glass of 600 feet focus; but he was never able to make any use of it. Hartsocker is even said to have made some of a still greater focal length; but this ingenious mechanic, finding it impossible to make use of object-glasses the focal distance of which was much less than this, when they were enclosed in a tube, contrived a method of using them without a tube, by fixing them at the top of a tree, a high wall, or the roof of a house.

365. Mr. Huygens also made considerable improvements in the method of using an objectglass without a tube. He placed it at the top of a very long pole, having previously enclosed it in a short tube, which was made to turn in all directions by a ball and socket. The axis of this tube he could command with a fine silken string so as to bring it into a line with the axis of another short tube which he held in his hand, and which contained the eye-glass. He thus could use object-glasses of the greatest magnifying power, at whatever altitude his object was, and even in the zenith, provided his pole was as long as his telescope; and, to adapt it to the view of objects of different altitudes, he had a contrivance by which he could raise or depress a stage that supported his object-glass at pleasure.

366. M. Auzout, in a paper read before the Royal Society, observed that the apertures which the object-glasses of refracting telescopes can bear with distinctness are in about a sub-duplicate proportion to their lengths; and upon this supposition he drew up a table of the apertures proper for object-glasses of a great variety of focal lengths, from four inches to 400 feet. Upon this occasion Dr. Hooke observed that the same glass will bear a greater or less aperture, according to the less or greater light of the object. If, for instance, he was viewing the sun, or Venus, or any of the fixed stars, he used smaller apertures; but if he wanted to view the moon by day light, or Saturn, Jupiter, or Mars, by night, he used a larger aperture.

367. But the merit of all those 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, supposing it possible to be made use of, could not be made to magnify with distinctness more than 1000 times; whereas a reflecting telescope, not exceeding nine or ten feet, will magnify 1200 times. It must be acknowledged,' says Dr. Smith in his Complete System of Optics, that Mr. James Gregory of Aberdeen was the first 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 specula he certainly proposed to the celebrated Descartes many years before Gregory's invention, though indeed in a manner so very unsatisfactory that Descartes, who had given particular attention to the improvement of the telescope, was so far from approving the proposal, that he endeavoured to Vor. XVI.

convince Mersennus of its fallacy. See TELE: scOPE. Dr. Smith had not seen two letters of Descartes to Mersennus on that subject.

368. Gregory, a young man of an uncommon genius, was led to the invention in seeking to correct two imperfections of the common telescope: the first was its too great length, which made it less manageable; the second, the incorrectness of the image. Mathematicians had demonstrated that a pencil of rays could not be collected in a single point of a spherical lens; and also that the image transmitted by such a lens would be in some degree incurvated. These inconveniences he believed would be obviated by substituting for the object-glass a metallic speculum, of a parabolic figure, to receive the image, and to reflect it towards a small speculum of the same metal; this again was to return the image to an eye-glass placed behind the great speculum, which for that purpose was to be forated in its centre. This construction he published in 1663 in his Optica Promota. But as Gregory was endowed with no mechanical dexterity, nor could find any workman capable of realising his invention, after some 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 colors, a reflecting telescope would never more have been thought of, considering the difficulty of the execution, and the small advantages that could accrue from it, deducible from the principles of optics then known.

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369. But Newton, whose genius for experimental knowledge was equal to that for geometry, happily interposed, and saved this noble invention from well nigh perishing in its infant state. He likewise, at an early period of life, had applied himself to the improvement of the telescope; but, imagining that Gregory's specula were neither very necessary nor likely to be executed, he began with prosecuting the views of Descartes, who aimed at making a more perfect image of an object by grinding lenses, not to the figure of a sphere, but to that of a conic section. While he was thus employed, three years after Gregory's publication, he happened to examine the colors formed by a prism, and, having by that simple instrument discovered the different refrangibility of the rays of light, he perceived that the errors of telescopes, arising from that cause alone, were some hundred times greater than those occasioned by the spherical figure of lenses. This circumstance forced Newton to fall into Gregory's track, and to turn his thoughts to reflectors. The different refrangibility of the rays of light,' he says, in a letter to Mr. Oldenburg, secretary to the Royal Society, dated February, 1672, 'made me take reflectors into consideration; and finding them regular, so that the angle of reflection of all sorts of rays was equal to the angle of incidence, I understood that, by their mediation, optic instruments might be brought to any degree of perfection imaginable, providing a reflecting substance could be found which would polish as finely as glass, and reflect as much light as glass transmits, and the art of communicating to it a parabolic figure be also obtained. Amidst these thoughts I was forced from Cambridge by

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the intervening plague, and it was more than two years before I proceeded further.' If Newton, then, was not the first inventor of the reflecting telescope, he was the main and effectual inventor. By his admirahle genius he fell upon this new property of light; and thereby found that all fenses, of whatever figure, would be affected more or less with such prismatic aberrations of the rays as would be an insuperable obstacle to the perfection of a dioptric telescope.

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370. About the end of 1668, or beginning of 1669, Newton, not relying on any artificer for making his specula, set about the work himself, and early in 1672 completed two small reflecting telescopes. In these he ground the great speculum into a spherical concave, but found himself unable to accomplish the parabolic form. In the letter that accompanied one of these instruments, which he presented to the Society, he writes, that though he then despaired 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 some measure be accomplished by mechanical devices.' 371. Not less did the difficulty appear to find a metallic substance that would be of a prope. hardness, have the fewest pores, and receive the smoothest polish a difficulty which he deemed almost insurmountable, when he considered that every irregularity in a reflecting surface would make the rays of light stray five or six times more out of their due course than the like irregularities in a refracting one. In another letter written soon after he tells the secretary that he was very sensible that metal reflects less light than glass transmits; but as he had found some metallic substances to be more strongly reflective than others, to polish better, and to be freer from tarnishing than others, so he hoped that there might in time be found out some substances much freer from these inconveniences than any yet known.' Newton therefore labored till he found a composition that answered in some degree, and presented a reflecting telescope to the Royal Society; from whom he received such thanks as were due to so curious and valuable a present. And Huygens, one of the greatest geniuses of the age, and a distinguished improver of the refractor, no sooner was informed by Mr. Oldenburg of the discovery, than he wrote an answer that it was an admirable telescope; and that Mr. Newton had well considered the advantage which a concave speculum had above convex glasses 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-glass of the same distance of focus, and consequently much more magnify in his way than by an ordinary telescope. Besides that by the reflector he avoided an inconvenience inseparable from object-glasses, which was the obliquity of both their surfaces, which vitiated the refraction of the rays that pass towards the sides of the glass, and did more hurt than men were aware of. Again, that by the mere reflection of the metalline speculum there were not so many rays lost as in glasses, which reflected a considerable quantity by each of their surfaces, and, besides, intercepted many of them by the

obscurity of their matter: that the main business would be to find a matter for this speculum that would bear as good a polish as glass.'

372. Mr. Huygens was not satisfied with thus expressing to the Society his high approbation of Newton's invention; but drew up a favorable account of the new telescope, which he caused to be published in the Journal de Sçavans for 1672, by which it was soon known over Europe. But how excellent soever the contrivance was, how well soever supported and announced to the public, yet whether it was that the artists were deterred by the difficulty and labor of the work, or that the discoveries even of a Newton were not to be exempted from the general fatality attending great and useful inventions, the making a slow and vexatious progress to the authors; the fact is that, excepting an unsuccessful attempt which the Society made, by employing an artificer to imitate the Newtonian construction, but upon a larger scale, and a disguised Gregorian telescope, set up by Cassegrain abroad as a rival to Newton's, and that in theory only (for it was never put in execution by the author) no reflector was heard of for nearly half a century after. But a reflecting telescope was at last produced to the world, of the Newtonian construction, by Dr. Hadley, which the author had the satisfaction to find executed in such a manner as left no room to fear that the invention would any longer continue in obscurity.

373. This memorable event was owing to the genius, dexterity, and application of Dr. Hadley, the inventor of the reflecting quadrant, another most valuable instrument. The two telescopes which Newton had made were but six inches long, were held in the hand for viewing objects, and in power were compared to a six feet refractor; whereas Hadley's was above five feet long, was provided with a well-contrived apparatus 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 specula we have, in the transactions of 1723, a complete description, with a figure of this telescope, together with that of the machine for moving it; but, by a strange omission, Newton's name is not once mentioned in that paper; so that any person not acquainted with the history of the invention, and reading that account only, might be apt to conclude that Hadley had been the sole inventor. The same celebrated artist, after finishing two telescopes of the Newtonian construction, accomplished a third in the Gregorian way; but, it would seem, less successfully, by Dr. Smith's declaring so strongly in favor of the other. Dr. Hadley instructed Mr. Molyneux and the Rev. Dr. Bradley; and when these gentlemen had made a sufficient proficiency in the art, being desirous that these telescopes should become more public, they liberally communicated to some of the principal instrument-makers of London the knowledge they had acquired from him. Such scholars soon advanced beyond their masters, and completed reflectors by other and better methods than what had been taught them.

374. Mr. James Short, as early as 1734, had signalised himselfat Edinburgh by his instruments

of this kind. Mr. Maclaurin wrote that year to Dr. Jurin, that Mr. Short, who had begun with making glass specula, 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 surpassed in perfection all that he bad seen of other workmen' He added, that Mr. Short's telescopes were all of the Gregorian construction; and that he had much improved that excellent invention.' This character of excellence Mr. Short maintained to the last; as he had been well grounded both in the geometrical and philosophical principles of optics, and upon the whole was a most intelligent person in whatever related to his profession. It was supposed he had fallen upon a method of giving the parabolic figure to his great speculum: a point of perfection which Hadley never attempted, either in his Newtonian or Gregorian telescope. Mr. Short indeed said he had acquired that faculty, but never would tell by what means he effected it; so that the secret of working that configuration died with that ingenious artist. Mr. Mudge, however, has lately realised the expectation of Sir Isaac Newton, who about 100 years ago presaged that the public would one day possess a parabolic speculum, not accomplished by mathematical rules, but by mechanical devices.

375. This desideratum was not the only want supplied by this gentleman: he taught us like wise a better composition of metals for the specula, how to grind them better, and how to give them a finer polish; and the polish, he remarks, was the most difficult and essential of the whole operation. In a word,' says Sir John Pringle, I am of opinion there is no optician in this great city (which has been so long and so justly renowned for ingenious and dextrous makers of every kind of mathematical instruments) so partial to his own abilities as not to acknowledge that, however some parts of the mechanical process now disclosed might have been known before any individuals of the profession, yet that Mr. Mudge has opened to them all some new and important lights, and upon the whole hath greatly improved the art of making reflecting telescopes.'

376. The late Rev. and ingenious John Edwards devoted much of his time to the improvement of reflecting telescopes, and brought them to such perfection that Dr. Maskelyne, the astronomer royal, found telescopes constructed by him to surpass in brightness, and other essentials, those of the same size made by the best artists in London. The chief excellence of his telescopes arises from the composition, which, from various trials on metals and semimetals, he discovered for the specula, 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 composition of specula, and for casting, grinding, and polishing them, were published, by order of the commissioners of longitude, at the end of the Nautical Almanac for 1787; to which is also annexed his account of the cause and cure of the tremors which particularly affect reflecting telescopes more than refracting ones,

together with remarks on the said tremors by Dr. Maskelyne.

377. But, in constructing reflecting telescopes of extraordinary magnifying powers, Dr. Herschel displayed skill and ingenuity surpassing all his predecessors. He made them from seven, ten, twenty, to even forty feet in length. Of its construction, magnifying powers, and the curious collection of machinery by which it is supported and moved from one part of the heavens to another, accounts will be found under TELESCOPE.

378. The greatest improvement in refracting telescopes hitherto made public is that of Mr. Dollond. But besides the obligations we are under to him for correcting the aberration of the rays of light in the focus of object-glasses, arising from their different refrangibility, he made another considerable improvement, viz. by correcting both this kind of aberration, and also that which arises from the spherical form of lenses, by an expedient of a very different nature, viz. increasing the number of eye-glasses. If any person, says he, would have the visual angle of a 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 eyeglass, will cause an aberration from the figure, in proportion to the cube of that angle; but, if two glasses are so proportioned and situated as that the refraction may be equally divided between them, they will each of them produce a refraction equal to half the required angle; and therefore the aberration, being in proportion to the cube of half the angle taken twice over, will be but a fourth part of that which is in proportion to the cube of the whole angle; because twice the cube of one is but one-fourth of the cube of two; so the aberration from the figure, where two eyeglasses are rightly proportioned, is but a fourth of what it must unavoidably be where the whole is performed by a single eye-glass. By the same reasoning, when the refraction is divided between three glasses, the aberration will be found to be but the ninth part of what would be produced from a single glass; because three times the cube of one is but one-ninth of the cube of three. Whence it appears that, by increasing the number of the eye-glasses, the indistinctness which is observed near the borders of the field of a telescope may be very much diminished, though not entirely taken away.

379. The method of correcting the errors arising from the different refrangibility of light is of a different consideration. For, whereas the errors from the figure can only be diminished in a certain proportion according to the number of glasses, in this they may be entirely corrected by the addition of only one glass; as we find in the astronomical telescope that two eye-glasses, rightly proportioned, will cause the edges of objects to appear free from colors, quite to the border of the field. Also in the day telescope, where no more than two eye-glasses are absolutely necessary for erecting the object, we find that, by the addition of a third rightly situated, the colors, which would otherwise make the image confused, are entirely removed. This, however, is to be understood with some limita