TELESCOPE
, an optical instrument which serves for discovering and viewing distant objects, either directly by glasses, or by reflection, by means of specula, or mirrors. Accordingly,
Telescopes are either refracting or reflecting; the former consisting of different lenses, through which the objects are seen by rays refracted through them to the eye; and the latter of specula, from which the rays are reflected and passed to the eye. The lens or glass turned towards the object, is called the object-glass; and that next the eye, the eye-glass; and when the Telescope consists of more than two lenses, all but that next the object are called eye-glasses.
The invention of the Telescope is one of the noblest and most useful these ages have to boast of: by means of it, the wonders of the heavens are discovered to us, and astronomy is brought to a degree of perfection which former ages could have no idea of. |
The discovery indeed was owing rather to chance than design; so that it is the good fortune of the discoverer, rather than his skill or ability, we are indebted to: on this account it concerns us the less to know, who it was that first hit upon this admirable invention. Be that as it may, it is certain it must have been casual, since the theory it depends upon was not then known.
John Baptista Porta, a Neapolitan, according to Wolfius, first made a Telescope, which he infers from this passage in the Magia Naturalis of that author, printed in 1560: “If you do but know how to join “the two (viz, the concave and convex glasses) rightly “together, you will see both remote and near objects, “much larger than they otherwise appear, and withal “very distinct. In this we have been of good help “to many of our friends, who either saw remote “things dimly, or near ones confusedly; and have “made them see every thing perfectly.”
But it is certain, that Porta did not understand his own invention, and therefore neither troubled himself to bring it to a greater perfection, nor ever applied it to celestial observation. Besides, the account given by Porta of his concave and convex lenses, is so dark and indistinct, that Kepler, who examined it by desire of the emperor Rudolph, declared to that prince, that it was perfectly unintelligible.
Thirty years afterwards, or in 1590, a Telescope 16 inches long was made, and presented to prince Maurice of Nassau, by a spectacle maker of Middleburg: but authors are divided about his name. Sirturus, in a treatise on the Telescope, printed in 1618, will have it to be John Lippersheim: and Borelli, in a volume expressly on the inventor of the Telescope, published in 1655, shews that it was Zacharias Jansen, or, as Wolsius writes it, Hansen.
Now the invention of Lippersheim is fixed by some in the year 1609, and by others in 1605: Fontana, in his Novæ Observationes Cælestium et Terrestrium Rerum, printed at Naples in 1646, claims the invention in the year 1608. But Borelli's account of the discovery of Telescopes is so circumstantial, and so well authenticated, as to render it very probable that Jansen was the original inventor.
In 1620, James Metius of Alcmaer, brother of Adrian Metius who was professor of mathematics at Franeker, came with Drebel to Middleburg, and there bought Telescopes of Jansen's children, who had made them public; and yet this Adr. Metius has given his brother the honour of the invention, in which too he is mistakenly followed by Descartes.
But none of these artificers made Telescopes of above a foot and a half: Simon Marius in Germany, and Galileo in Italy, it is said, first made long ones fit for celestial observations; though, from the recently discovered astronomical papers of the celebrated Harriot, author of the Algebra, it appears that he must have made use of Telescopes in viewing the solar maculæ, which he did quite as early as they were observed by Galileo. Whether Harriot made his own Telescopes, or whether he had them from Holland, does not appear: it seems however that Galileo's were made by himself; for Le Rossi relates, that Galileo, being then at Venice, was told of a sort of optic glass made in Holland, which brought objects nearer: upon which, setting himself to think how it should be, he ground two pieces of glass into form as well as he could, and fitted them to the two ends of an organpipe; and with these he shewed at once all the wonders of the invention to the Venetians, on the top of the tower of St. Mark. The same author adds, that from this time Galileo devoted himself wholly to the improving and perfecting the Telescope; and that he hence almost deserved all the honour usually done him, of being reputed the inventor of the instrument, and of its being from him called Galileo's tube. Galileo himself, in his Nuncius Sid<*>us, published in 1610, acknowledges that he first heard of the instrument from a German; and that, being merely informed of its effects, first by common report, and a few days after by letter from a French gentleman, James Badovere, at Paris, he himself discovered the construction by considering the nature of refraction. He adds in his Saggiatore, that he was at Venice when he heard of the effects of prince Maurice's instrument, but nothing of its construction; that the first night after his return to Padua, he solved the problem, and made his instrument the next day, and soon after presented it to the Doge of Venice, who, in honour of his grand invention, gave him the ducal letters, which settled him for life in his lectureship, at Padua, and doubled his salary, which then became treble of what any of his predecessors had enjoyed before. And thus Galileo may be considered as an inventor of the Telescope, though not the first inventor.
F. Mabillon indeed relates, in his travels through Italy, that in a monastery of his own order, he saw a manuscript copy of the works of Commestor, written by one Conradus, who lived in the 13th century; in the 3d page of which was seen a portrait of Ptolomy, viewing the stars through a tube of 4 joints or draws: but that father does not say that the tube had glasses in it. Indeed it is more than probable, that such tubes were then used for no other purpose but to defend and direct the sight, or to render it more distinct, by singling out the particular object looked at, and shutting out all the foreign rays reflected from others, whose proximity might have rendered the image less precise. And this conjecture is verified by experience; for we have often observed that without a tube, by only looking through the hand, or even the fingers, or a pinhole in a paper, the objects appear more clear and distinct than otherwise.
Be this as it may, it is certain that the optical principles, upon which Telescopes are founded, are contained in Euclid, and were well known to the ancient geometricians; and it has been for want of attention to them, that the world was so long without that admirable invention; as doubtless there are many others lying hid in the same principles, only waiting for reflection or accident to bring them forth.
To the foregoing abstract of the history of the invention of the Telescope, it may be proper to add some particulars relating to the claims of our own celebrated countryman, friar Bacon, who died in 1294. Mr. W. Molyneux, in his Dioptrica Nova, pa. 256, declares his opinion, that Bacon did perfectly well understand all sorts of optic glasses, and knew likewise the | way of combining them, so as to compose some such instrument as our Telescope: and his son, Samuel Molyneux, asserts more positively, that the invention of Telescopes, in its first original, was certainly put in practice by an Englishman, friar Bacon; although its first application to astronomical purposes may probably be ascribed to Galileo. The passages to which Mr. Molyneux refers, in support of Bacon's claims, occur in his Opus Majus, pa. 348 and 357 of Jebb's edit. 1773. The first is as follows: Si vero non sint corpora plana, per quæ visus videt, sed sphæria, tunc est magna diversitas; nam vel concavitas corporis est versus oculum vel convexitas: whence it is inferred, that he knew what a concave and a convex glass was. The second is comprised in a whole chapter, where he says, De visione fracta majora sunt; nam de facili patet per canones supra dictos, quod maxima possunt apparere minima, et e contra, et longe distantia videbuntur propinquissime, et e converso. Nam possumus sic figurare perspicua, et taliter ea ordinare respectu nostri visus et rerum, quod frangentur radii, et flectentur quorsumcunque voluerimus, ut sub quocunque angulo voluerimus, videbimus rem prope vel longe, &c. Sic etiam faceremus solem et lunam et stellas descendere secundum apparentiam hic inferius, &c: that is, Greater things than these may be performed by refracted vision; for it is easy to understand by the canons above mentioned, that the greatest things may appear exceeding small, and the contrary; also that the most remote objects may appear just at hand, and the converse; for we can give such figures to transparent bodies, and dispose them in such order with respect to the eye and the objects, that the rays shall be refracted and bent towards any place we please; so that we shall see the object near at hand or at a distance, under any angle we please, &c. So that thus the sun, moon, and stars may be made to descend hither in appearance, &c. Mr. Molyneux has also cited another passage out of Bacon's Epistle ad Parisiensem, of the Secrets of Art and Nature, cap. 5, to this purpose, Possunt etiam sic figurari perspicua, ut longissime posita appareant propinqua, et è contrario; ita quod ex incredibili distantia legeremus literas minutissimas, et numeraremus res quantumquo parvas, et stellas faceremus apparere quo vellemus: that is, 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 appear as near as we please.
Moreover, Doctor Jebb, in the dedication of his edition of the Opus Majus, produces a passage from a manuscript, to shew that Bacon actually applied Telescopes to astronomical purposes: Sed longe magis quam hæc, says he, oporterel homines haberi, qui bene, immo optime, scirent perspectivam et instrumenta ejus—quia instrumenta astronomia non vadunt nisi per visionem secundum leges istius scientiæ.
From these passages, it is not unreasonable to conclude, that Bacon had actually combined glasses so as to have produced the effects which he mentions, though he did not complete the construction of Telescopes. Dr. Smith, however, to whose judgment particular deference is due, is of opinion that the celebrated friar wrote hypothetically, without having made any actual trial of the things he mentions: to which purpose he observes, that this author does not assert one fingle trial or observation upon the sun or moon, or any thing else, though he mentions them both: on the other hand, he imagines some effects of Telescopes that cannot possibly be performed by them. He adds, that persons unexperienced in looking through Telescopes expect, in viewing any object, as for instance the face of a man, at the distance of one hundred yards, through a Telescope that magnifies one hundred times, that it will appear much larger than when they are close to it: this he is satisfied was Bacon's notion of the matter; and hence he concludes that he had never looked through a Telescope.
It is remarkable that there is a passage in Thomas Digges's Stratioticos, pa. 359, where he affirms that his father, Leonard Digges, among other curious practices, had a method of discovering, by perspective glasses set at due angles, all objects pretty far distant that the sun shone upon, which lay in the country round about; and that this was by the help of a manuscript book of Roger Bacon of Oxford, who he conceived was the only man besides his father (since Archimedes) who knew it. This is the more remarkable, because the Stratioticos was first printed in 1579, more than 30 years before Metius or Galileo made their discovery of those glasses; and therefore it has hence been thought that Roger Bacon was the first inventor of Telescopes, and Leonard Digges the next reviver of them. But from what Thomas Digges says of this matter, it would seem that the instrument of Bacon, and of his father, was something of the nature of a camera obscura, or, if it were a Telescope, that it was of the reflecting kind; although the term perspective glass seems to favour a contrary opinion.
There is also another passage to the same effect in the preface to the Pantometria of Leonard Digges, but published by his son Thomas Digges, some time before the Stratioticos, and a second time in the year 1591. The passage runs thus: My father by his continuall painfull practises, assisted with demonstrations mathematical, was able, and sundrie times hath by Proportional Glasses duely situate in convenient angles, not only discovered things farre off, read letters, numbered peeces of money with the very coyne and superscription thereof, cast by some of his frecnds of purpose upon downes in open fields, but also seven myles off declared what hath beene doone at that instant in private places: He hath also sundrie times by the sunne beames fixed (should be fired) powder, and dischargde ordinance halfe a mile and more distante, &c.
But to whomsoever we ascribe the honour of first inventing the Telescope, the rationale of this admirable instrument, depending on the refraction of light in passing through mediums of different forms, was first explained by the celebrated Kepler, who also pointed out methods of constructing others, of superior powers, and more commodious application, than that first used: though something of the same kind, it is said, was also done by Maurolycus, whose treatise De Lumine et Umbra was published in 1575.
The Principal Effects of Telescopes, depend upon this plain maxim, 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 in certain directions, in the same manner as they did from the corresponding points in the objects themselves. In fact therefore, 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 give the eye some assistance for 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 in the same situation. This is done by means of an eye-glass, which so refracts the pencils of rays, as that they may afterwards be brought to their several foci, by the natural humours of the eye. But if the eye had been so formed as to be able to see the image, with sufficient distinctness, at the same distance, without an eye-glass, it would appear to him as much magnified, as it does to another person who makes use of a glass for that purpose, though he would not in all cases have so large a field of view.
Although no image be actually formed by the foci of the pencil without the eye, yet if, by the help of an eye-glass, the pencils of rays shall enter the pupil, just as they would have done from any place without the eye, the visual angle will be the same as if an image had been actually formed in that place. Priestley's History of Light &c, pa. 69, &c.
As to the Grinding of Telescopic Glasses, the first persons who distinguished themselves in that way, were two Italians, Eustachio Divini at Rome, and Campani at Bologna, whose fame was much superior to that of Divini, or that of any other person of his time; though Divini himself pretended, that in all the trials that were made with their glasses, his of a great focal distance performed better than those of Campani, and that his rival was not willing to try them fairly, viz, with equal eye-glasses. It is however generally supposed, that Campani really excelled Divini, both in the goodness and the focal length of his object-glasses.
It was with Campani's Telescopes that Cassini discovered the nearest satellites of Saturn. They were made at the express desire of Lewis XIV, and were of 86, 100, and 136, Paris feet focal length.
Campani's laboratory was purchased, after his death, by pope Benedict XIV, who made a present of it to the academy at Bologna called the Institute; and by the account which Fougeroux has given, we learn that (except a machine which Campani constructed, to work the basons on which he ground his glasses) the goodness of his lenses depended upon the clearness of his glass, his Venetian tripoli, the paper with which he polished his glasses, and his great skill and address as a workman. It does not appear that he made many lenses of a very great focal distance. Accordingly Dr. Hook, who probably speaks with the partiality of an Englishman, says that some glasses, made by Divini and Campani, of 36 and 50 feet focal distance, did not excel Telescopes of 12 or 15 feet made in England. He adds, that Sir Paul Neilli made Telescopes of 36 feet, pretty good; and one of 50, but not of proportionable goodness.
Afterwards, Mr. Reive first, and then Mr. Cox, who were the most celebrated in England, as grinders of optic glasses, made some good Telescopes of 50 and 60 feet focal distance; and Mr. Cox made one of 100, but how good Dr. Hook could not assert. Borelli also in Italy, made object-glasses of a great focal length, one of which he presented to the Royal Society. But, with respect to the focal length of Telescopes, these and all others were far exceeded by those of Auzout, who made one object-glass of 600 feet focus; but he was never able to manage it, so as to make any use of it. And Hartsoeker, it is said, made some of a still greater focal length. Philos. Trans. Abr. vol. i, p. 193. Hook's Exper. by Derham, p. 261. Priestley as above, p. 211. See Grinding.
Telescopes are of several kinds, distinguished by the number and form of their lenses, or glasses, and denominated from their particular uses &c: such are the terrestrial or land Telescope, the celestial or astronomical Telescope; to which may be added, the Galilean or Dutch Telescope, the reflecting Telescope, the refracting Telescope, the aërial Telescope, achromatic Telescope, &c.
Galileo's, or the Dutch Telescope, is one consisting of a convex object-glass, and a concave eyeglass.
This is the most ancient form of any, being the only kind made by the inventors, Galileo, &c. or known, before Huygens. The first Telescope, constructed by Galileo, magnified only 3 times; but he soon made another, which magnified 18 times: and afterwards, with great trouble and expence, he constructed one that magnified 33 times; with which he discovered the satellites of Jupiter, and the spots of the sun. The construction, properties. &c, of it, are as follow:
Construction of Galileo's, or the Dutch Telescope. In a tube prepared for the purpose, at one end is fitted a convex object lens, either a plain convex, or convex on both sides, but a segment of a very large sphere: at the other end is fitted an eye-glass, concave on both sides, and the segment of a less sphere, so disposed as to be at the distance of the virtual focus before the image of the convex lens.
Let AB (fig. 10, pl. 23) be a distant object, from every point of which pencils of rays issue, and falling upon the convex glass DE, tend to their foci at FSG. But a concave lens HI (the focus of which is at FG) being interposed, the converging rays of each pencil are made parallel when they reach the pupil; so that by the refractive humours of the eye, they can easily be brought to a focus on the retina at PRQ. Also the pencils themselves diverging, as if they came from X, MXO is the angle under which the image will appear, which is much larger than the angle under which the object itself would have appeared. Such then is the Telescope that was at first discovered and used by philosophers: the great inconvenience of which is, that the field of view, which depends, not on the breadth of the eye-glass, as in the astronomical Telescope, but upon the breadth of the pupil of the eye, is exceedingly small: for since the pencils of the rays enter the eye very much diverging from one another, but few of them can be intercepted by the pupil; and this inconvenience increases with the magnifying power of the Telescope, so that philosophers may now well wonder | at the patience and address with which Galileo and others, with such an instrument, made the discoveries they did. And yet no other Telescope was thought of for many years after the discovery. Descartes, who wrote 30 years after, mentions no other as actually constructed, though Kepler had suggested some. Hence,
1. In an instrument thus framed, all people, except myopes, or short-sighted persons, must see objects distinctly in an erect situation, and increased in the ratio of the distance of the virtual focus of the eyeglass, to the distance of the focus of the object glass.
2. But for myopes to see objects distinctly through such an instrument, the eye-glass must be set nearer the object-glass, so that the rays of each pencil may not emerge parallel, but may fall diverging upon the eye; in which case the apparent magnitude will be altered a little, though scarce sensibly.
3. Since the focus of a plano-convex object lens, and the vertical focus of a plano-concave eye-lens, are at the distance of the diameter; and the focus of an object-glass convex on both sides, and the vertical focus of an eye-glass concave on both sides, are at the distance of a semidiameter; if the object-glass be planoconvex, and the eye-glass plano-concave, the Telescope will increase the diameter of the object, in the ratio of the diameter of the concavity to that of the convexity: if the object-glass be convex on both sides, and the eye-glass concave on both sides, it will magnify in the ratio of the semidiameter of the concavity to that of the convexity: if the object-glass be plano-convex, and the eye-glass concave on both sides, the semidiameter of the object will be increased in the ratio of the diameter of the convexity to the semidiameter of the concavity: and lastly, if the object-glass be convex on both sides, and the eye-glass plano-concave, the increase will be in the ratio of the diameter of the concavity to the semidiameter of the convexity.
4. Since the ratio of the semidiameters is the same as that of the diameters, Telescopes magnify the object in the same manner, whether the object-glass be planoconvex, and the eye-glass plano-concave; or whether the one be convex on both sides, and the other concave on both.
5. Since the semidiameter of the concavity has a less ratio to the diameter of the convexity than its diameter has, a Telescope magnifies more if the objectglass be plano-convex, than if it be convex on both sides. The case is the same if the eye-glass be concave on both sides, and not plano-concave.
6. The greater the diameter of the object-glass, and the less that of the eye-glass, the less ratio has the diameter of the object, viewed with the naked eye, to its semidiameter when viewed with a Telescope, and consequently the more is the object magnified by it.
7. Since a Telescope exhibits so much a less part of the object, as it increases its diameter more, for this reason, mathematicians were determined to look out for another Telescope, after having clearly found the imperfection of the first, which was discovered by ehance. Nor were their endeavours vain, as appears from the astronomical Telescope described below.
If the semidiameter of the eye-glass have too small a ratio to that of the object-glass, an object through the Telescope will not appear sufficiently clear, because the great divergency of the rays will occasion the several pencils representing the several points of the object on the retina, to consist of too few rays.
It is also found that equal object-lenses will not bear the same eye-lenses, if they be differently transparent, or if there be a difference in their polish; a less transparent object-glass, or one less accurately ground, requiring a more spherical eye-glass than another more transparent, &c.
Hevelius recommends an object-glass convex on both sides, whose diameter is 4 feet; and an eye-glass concave on both sides, whose diameter is 4 1/2 tenths of afoot. An object-glass, equally convex on both sides, whose diameter is 5 feet, he observes, will require an eye-glass of 5 1/2 tenths; and adds, that the same eye-glass will also serve an object-glass of 8 or 10 feet.
Hence, as the distance between the object-glass and eye-glass is the difference between the distance of the vertical focus of the eye-glass, and the distance of the focus of the object glass; the length of the telescope is had by subtracting that from this. That is, the length of the Telescope is the difference between the diameters of the object-glass and eye-glass, if the former be plano-convex, and the latter plano-concave; or the difference between the semidiameters of the object-glass and eye-glass, if the former be convex on both sides, and the latter concave on both; or the difference between the semidiameter of the object-glass and the diameter of the eye-glass, if the former be convex on both sides, and the latter planoconcave; or lastly the difference between the diameter of the object-glass and the semidiameter of the eyeglass, if the former be plano-convex, and the latter concave on both sides. Thus, for instance, if the diameter of an object-glass, convex on both sides, be 4 feet, and that of an eye-glass, concave on both sides, be 4 1/2 tenths of a foot; then the length of the Telescope will be 1 foot and 7 1/2 tenths.
Astronomicel Telescope; this is one that consists of an object-glass, and an eye-glass, both convex. It is so called from being wholly used in astronomical observations.
It was Kepler who first suggested the idea of this Telescope; having explained the rationale, and pointed out the advantages of it in his Catoptrics, in 1611. But the first person who actually made an instrument of this construction, was father Scheiner, who has given a description of it in his Rosa Ursina, published in 1630. To this purpose he says, If you insert two similar convex lenses in a tube, and place your eye at a convenient distance, you will see all terrestrial objects, inverted indeed, but magnified and very distinct, with a considerable extent of view. He afterwards subjoined an account of a Telescope of a different construction, with two convex eye-glasses, which again reverses the images, and makes them appear in their natural position. Father Reita however soon after proposed a better construction, using three eye-glasses instead of two.
Construction of the Astronomical Telescope. The tube being prepared, an object-glass, either plano-con- | vex, or convex on both sides, but a segment of a large sphere, is fitted in at one end; and an eye-glass, convex on both sides, which is the segment of a small sphere, is fitted into the other end; at the common distance of the foci.
Thus the rays of each pencil issuing from every point of the object ABC, (fig. 3 pl. 30) passing through the object-glass DEF, become converging, and meet in their foci at IHG, where an image of the object will be formed. If then another convex lens KM, of a shorter focal length, be so placed, as that its focus shall be in IHG, the rays of each pencil, after passing through it, will become nearly parallel, so as to meet upon the retina, and form an enlarged image of the object at RST. If the process of the rays be traced, it will presently be perceived that this image must be inverted. For the pencil that issues from A, has its focus in G, and again in R, on the same side with A. But as there is always one inversion in simple vision, this want of inversion produces just the reverse of the natural appearance. The field of view in this Telescope will be large, because all the pencils that can be received on the surface of the lens KM, being converging after passing through it, are thrown into the pupil of the eye, placed in the common intersection of the pencils at P.
Theory of the Astronomical Telescope.—An eye placed near the focus of the eye-glass, of such a Telescope, will see objects distinctly, but inverted, and magnified in the ratio of the distance of the focus of the eye-glass to the distance of the focus of the objectglass.
If the sphere of concavity in the eye-glass of the Galilean Telescope, be eqnal to the sphere of convexity in the eye-glass of another Telescope, their magnifying power will be the same. The concave glass however being placed between the object-glass and its focus, the Galilean Telescope will be shorter than the other, by twice the focal length of the eye-glass. Consequently, if the length of the Telescopes be the same; the Galilean will have the greater magnifying power. Vision is also more distinct in these Telescopes, owing in part perhaps to there being no intermediate image between the eye and the object. Besides, the eye-glass being very thin in the centre, the rays will be less liable to be distorted by irregularities in the substance of the glass. Whatever be the cause, we can sometimes see Jupiter's satellites very clearly in a Galilean Telescope, of 20 inches or 2 feet long, when one of 4 or 5 feet, of the common sort, will hardly make them visible.
As the astronomical Telescope exhibits objects inverted, it serves commodiously enough for observing the stars, as it is not material whether they be seen erect or inverted; but for terrestrial objects it is much less proper, as the inverting often prevents them from being known. But if a plane well-polished metal speculum, of an oval figure, and about an inch long, and inclined to the axis in an angle of 45°, be placed bebehind the eye-glass; then the eye, conveniently placed, will see the image, hence reflected, in the same magnitude as before, but in an erect situation; and therefore, by the addition of such a speculum, the astronomical Telescope is thus rendered sit to observe terrestrial objects.
Since the focus of the glass convex on both sides is distant from the glass itself a semidiameter, and that of a plano-convex glass, a diameter; if the objectglass be convex on both sides, the Telescope will magnify the semidiameter of the object, in the ratio of the diameter of the eye-glass to the diameter of the object-glass; but if the object-glass be a plano-convex, in the ratio of the semidiameter of the eye-glass to the diameter of the object-glass. And therefore a Telescope magnifies more if the object-glass be a planoconvex, than if convex on both sides. And for the same reason, a Telescope magnifies more when the eyeglass is convex on both sides, than when it is planoconvex.
A Telescope magnifies the more, as the object-glass is a segment of a great sphere, and the eye-glass of a less one. And yet the eye-glass must not be too small in respect of the object-glass; for if it be, it will not refract rays enough to the eye from each point of the object; nor will it separate sufficiently those that come from different points; by which means the vision will be rendered obscure and confused.—De Chales observes, that an object-lens of 2 1/4 feet will require an eye-glass of 1 1/2 tenth of a foot; and an object-glass of 8 or 10 feet, an eye-glass of 4 tenths; in which he is confirmed by Eustachio Divini.
To shorten the Astronomical Telescope; that is, to construct a Telescope so, as that, though shorter than the common one, it shall magnify as much.
Having provided a drawing tube, fit in it an objectlens EO which is a segment of a moderate sphere: let the first eye-glass BD be concave on both sides, and so placed in the tube, as that the focus of the objectglass A may be behind it, but nearer to the centre of the concavity G: then will the image be thrown in Q, so as that GA : GI : : AB : QI. Lastly, sit in another object-glass, convex on both sides, and a segment of a smaller sphere, so as that its focus may be in Q.
This Telescope will magnify the diameter of the object more than if the object-glass were to represent its image at the same distance EQ; and consequently a shorter Telescope, constructed this way, is equivalent to a longer in the common way. See Wolfius Elem. Math. vol. 3, p. 245.
Sir Isaac Newton furnishes us with another method of constructing the Telescope, in his catoptrical or reflecting Telescope, the construction of which is given below. See Achromatic Telescope.
Aërial Telescope, a kind of astronomical Telescope, the lenses of which are used without a tube. In strictness however, the aërial Telescope is rather a particular manner of mounting and managing long | Telescopes for celestial observation in the nighttime, by which the trouble of long unwieldy tubes is saved, than a particular kind of Telescope; and the contrivance was one of Huygens's. This invention was successfully practised by the inventor himself and others, particularly with us by Dr. Pound and Dr. Bradley, with an object-glass of 123 feet focal distance, and an apparatus belonging to it, made and presented by Huygens to the Royal Society, and described in his Astroscopia Compendiaria Tubi Optici Molimine Liberata, printed at the Hague in 1684.
The principal parts of this Telescope may be comprehended from a view of fig. 4, pl. 30, where AB is a long pole, or a mast, or a high tree, &c, in a groove of which slides a piece that carries a small tube LK in which is fixed an object glass; which tube is connected by a fine line, with another small tube OQ, which contains the eye-glass, &c.
La Hire contrived a little machine for managing the object-glass which is described Mem. de l'Acad. 1715. See Smith's Optics, book 3, chap. 10.
Hartsoeker, who made Telescopes of a very considerable focal length, contrived a method of using them without a tube, by fixing them to the top of a tree, a high wall, or the roof of a house. Miscel. Berol. vol. 1, p. 261.
Huygens's great Telescope, with which Saturn's true face, and one of his satellites were first discovered, consists of an object-glass of 12 feet, and an eye-glass of a little more than 3 inches; though he frequently used a Telescope of 23 feet long, with two eye-glasses joined together, each 1 1/2 inch diameter; so that the two were equal to one of 3 inches.
The same author observes, that an object-glass of 30 feet requires an eye-glass of 3 3/10 inches; and has given a table of proportions for constructing astronomical Telescopes, an abridgment of which is as follows:
| Dist. of | Diameter | Dist. of | Power or |
| Foc. of | of | Foc. of | Magnitude |
| Obj. Glass. | Apert. | Eye-glass. | of Diam. |
| Feet. | Inches | Inches | |
| and Decim. | and Decim. | ||
| 1 | 0.55 | 0.61 | 20 |
| 2 | 0.77 | 0.85 | 28 |
| 3 | 0.95 | 1.05 | 34 |
| 4 | 1.09 | 1.20 | 40 |
| 5 | 1.23 | 1.35 | 44 |
| 6 | 1.34 | 1.47 | 49 |
| 7 | 1.45 | 1.60 | 53 |
| 8 | 1.55 | 1.71 | 56 |
| 9 | 1.64 | 1.80 | 60 |
| 10 | 1.73 | 1.90 | 63 |
| 15 | 2.12 | 2.33 | 77 |
| 20 | 2.45 | 2.70 | 89 |
| 25 | 2.74 | 3.01 | 100 |
| 30 | 3.00 | 3.30 | 109 |
| 40 | 3.46 | 3.81 | 120 |
| Dist. of | Diameter | Dist. of | Power or |
| Foc. of | of | Foc. of | Magnitude |
| Object-glass. | Apert. | Eye-glass. | of Diam. |
| Feet. | Inches | Inches | |
| and Decim. | and Decim. | ||
| 50 | 3.87 | 4.26 | 141 |
| 60 | 4.24 | 4.66 | 154 |
| 70 | 4.58 | 5.04 | 166 |
| 80 | 4.90 | 5.39 | 178 |
| 90 | 5.20 | 5.72 | 189 |
| 100 | 5.49 | 6.03 | 200 |
| 120 | 6.00 | 6.60 | 218 |
| 140 | 6.48 | 7.12 | 235 |
| 160 | 6.93 | 7.62 | 252 |
| 180 | 7.35 | 8.09 | 267 |
| 200 | 7.75 | 8.53 | 281 |
| 220 | 8.12 | 8.93 | 295 |
| 240 | 8.48 | 9.33 | 308 |
| 260 | 8.83 | 9.71 | 321 |
| 280 | 9.16 | 10.08 | 333 |
| 300 | 9.49 | 10.44 | 345 |
| 400 | 10.95 | 12.05 | 400 |
| 500 | 12.25 | 13.47 | 445 |
| 600 | 13.42 | 14.76 | 488 |
Dr. Smith (Rem. p. 78) observes, that the magnifying powers of this table are not so great as Huygens himself intended, or as the best object-glasses now made will admit of. For the author, in his Astroscopia Compendiaria, mentions an object-glass of 34 feet focal distance, which, in astronomical observations, bore an eye-glass of 2 1/2 inches focal distance, and consequently magnified 163 times. According to this standard, a Telescope of 35 feet ought to magnify 166 times, and of 1 foot 28 times; whereas the table allows but 118 times to the former, and but 20 to the latter. Now 166/118 or 28/20 = 1.4; by which if we multiply the numbers in the given column of magnifying powers, we shall gain a new column, shewing how much those object-glasses ought to magnify if wrought up to the perfection of this standard.
The new apertures and eye-glasses must also be taken in the same proportions to one another, as the old ones have in the table; or the eye-glasses may be found by dividing the length of each Telescope by its magnifying power. And thus a new table may be easily made for this or any other more perfect standard when offered.
The rule for computing this table depends on the following theorem, viz, that in refracting Telescopes of different lengths, a given object will appear equally bright and equally distinct, when their linear apertures and the focal distances of their eye-glasses are severally in a subduplicate ratio of their lengths, or focal distances of their object-glasses; and then also the breadth of their apertures will be in the subduplicate ratio of their lengths.
The rule is this: Multiply the number of feet in the focal distance of any proposed object-glass by 3000, and the square-root of the product will give the breadth of its aperture in centesms, or 100th parts of an inch <*> | that is, √(3000F) is the breadth of the aperture in centesms of an inch, where F is the focal distance of the object-glass in feet. Also, the same breadth of the aperture increased by the 10th part of itself, gives the focal distance of the eye-glass in centesms of an inch. And the magnifying powers are as the breadths of the apertures.
If, in different Telescopes, the ratio between the object-glass and eye-glass be the same, the object will be magnified the same in both. Hence some may conclude the making of large Telescopes a needless trouble. But it must be remembered, that an eye-glass may be in a less ratio to a greater object-glass than to a smaller: thus, for example, in Huygens's Telescope of 25 feet, the eye-glass is 3 inches: now, keeping this proportion in a Telescope of 50 feet, the eye-glass should be 6 inches; but the table shews that 4 1/2 are sufficient. Hence, from the same table it appears, that a Telescope of 50 feet magnifies in the ratio of 1 to 141; whereas that of 25 feet only magnifies in the ratio of 1 to 100.
Since the distance of the lens is equal to the aggregate of the distances of the foci of the object and eye-glasses; and since the focus of a glass convex on each side is a semidiameter's distance from the lens, and that of a plano-convex at a diameter's distance from the same; the length of a Telescope is equal to the aggregate of the semidiameters of the lenses, if the object-glass be convex on both sides; and to the sum of the semidiameter of the eye-glass and the whole diameter of the object glass, if the object-glass be a plano-convex.
But as the diameter of the eye-glass is very small in respect of that of the object-glass, the length of the Telescope is usually estimated from the distance of the object-glass; i. e. from its semidiameter if it be convex on both sides, or its whole diameter if plano-convex. Thus, a Telescope is said to be 12 feet, if the semidiameter of the object-glass, convex on both sides, be 12 feet, &c.
Since myopes see near objects best; for them, the eye-glass is to be removed nearer to the object-glass, that the rays refracted through it may be the more diverging.
To take in the larger field at one view, some make use of two eye-glasses, the foremost of which is a segment of a larger sphere than that behind; to this it must be added, that if two lenses be joined immediately together, so as the one may touch the other, the focus is removed to double the distance which that of one of them would be at.
Land Telescope, or Day Telescope, is one adapted for viewing objects in the day-time, on or about the earth. This contains more than two lenses, usually it has a convex object-glass, and three convex eye-glasses; exhibiting objects erect, yet different from that of Galileo.
In this Telescope, after the rays have passed the first eye-glass HI (fig. 2, pl. 30), as in the former construction, instead of being there received by the eye, they pass on to another equally convex lens, situated at twice its focal distance from the other, so that the rays of each pencil, being parallel in that whole interval, those pencils cross one another in the common focus, and the rays constituting them are transmitted parallel to the second eye-glass LM; after which the rays of each pencil converge to other foci at NO, where a second image of the object is formed, but inverted with respect to the former image in EF. This image then being viewed by a third eye-glass QR, is painted upon the retina at XYZ, exactly as before, only in a contrary position.
Father Reita was the author of this construction; which is effected by fitting in at one end of a tube an object-glass, which is either convex on both sides, or plano-convex, and a segment of a large sphere; to this add three eye-glasses, all convex on both sides, and segments of equal spheres; disposing them in such a manner as that the distance between any two may be the aggregate of the distances of their foci. Then will an eye applied to the last lens, at the distance of its focus, see objects very distinctly, erect, and magnified in the ratio of the distance of the focus of one eye-glass, to the distance of the focus of the objectglass.
Hence, 1. An astronomical Telescope is easily converted into a Land Telescope, by using three eyeglasses for one; and the Land Telescope, on the contrary, into an astronomical one, by taking away two eye-glasses, the faculty of magnifying still remaining the same.
2. Since the distance of the eye-glasses is very small, the length of the Telescope is much the same as if you only used one.
3. The length of the Telescope is found by adding five times the semidiamer of the eye-glasses, to the diameter of the object-glass when this is a planoconvex, or to its semidiameter when convex on both sides.
Huygens first observed, both in the astronomical and Land Telescope, that it contributes considerably to the perfection of the instrument, to have a ring of wood or metal, with an aperture, a little less than the breadth of the eye-glass, sixed in the place where the image is found to radiate upon the lens next the eye: by means of which, the colours, which are apt to disturb the clearness and distinctness of the object, are prevented, and the whole compass taken in at one view, perfectly defined.
Some make Land Telescopes of three lenses, which yet represent objects erect, and magnified as much as the former. But such Telescopes are subject to very great inconveniences, both as the objects in them are tinged with false colours, and as they are distorted about the margin.
Some again use five lenses, and even more; but as some parts of the rays are intercepted in passing every lens, objects are thus exhibited dim and feeble.
Telescopes of this kind, longer than 20 feet, will be of hardly any use in observing terrestrial objects, on account of the continual motion of the particles of the atmosphere, which these powerful Telescopes render visible, and give a tremulous motion to the objects themselves.
The great length of dioptric Telescopes, adapted to any important astronomical purpose, rendered them extremely inconvenient for use; as it was necessary to increase their length in no less a proportion than the | duplicate of the increase of their magnifying power: so that, in order to magnify twice as much as before, with the same light and distinctness, the Telescope required to be lengthened 4 times; and to magnify thrice as much, 9 times the length, and so on. This unwieldiness of refracting Telescopes, possessing any considerable magnifying power, was one cause, why the attention of astronomers, &c, was directed to the discovery and construction of reflecting Telescopes. And indeed a refracting Telescope, even of 1000 feet focus, supposing it possible to make use of such an instrument, could not be made to magnify with distinctness more than 1000 times; whereas a reflecting Telescope, of 9 or 10 feet, will magnify 12 hundred times. The perfection of refracting Telescopes, it is well known, is very much limited by the aberration of the rays of light from the geometrical focus: and this arises from two different causes, viz, from the different degrees of refrangibility of light, and from the figure of the sphere, which is not of a proper curvature for collecting the rays in a single point. Till the time of Newton, no optician had imagined that the object glasses of Telescopes were subject to any other error beside that which arose from their spherical figure, and therefore all their efforts were directed to the construction of them, with other kinds of curvature: but that author had no sooner demonstrated the different refraugibility of the rays of light, than he discovered in this circumstance a new and a much greater cause of error in Telescopes. Thus, since the pencils of each kind of light have their foci in different places, some nearer and some farther from the lens, it is evident that the whole beam cannot be brought into any one point, but that it will be drawn the nearest to a point in the middle place between the focus of the most and least refrangible rays; so that the focus will be a circular space of a considerable diameter. Newton shews that this space is about the 55th part of the aperture of the Telescope, and that the focus of the most refrangible rays is nearer to the object-glass than the focus of the least refrangible ones, by about the 27 1/2 part of the distance between the object-glass, and the focus of the mean refrangible rays. But he says, that if the rays flow from a lucid point, as far from the lens on one side as their foci are on the other, the focus of the most refrangible rays will be nearer to the lens than that of the least refrangible, by more than the 14th part of the whole distance. Hence, he concludes, that if all the rays of light were equally refrangible, the error in Telescopes, arising from the spherical figure of the glass, would be many hundred times less than it now is; because the error arising from the spherical figure of the glass, is to that arising from the different refrangibility of the rays of light, as 1 to 5449. See Aberration.
Upon the whole he observes, that it is a wonder that Telescopes represent objects so distinctly as they do. The reason of which is, that the dispersed rays are not scattered uniformly over all the circular space above-mentioned, but are infinitely more dense in the centre than in any other part of the circle; and that in the way from the centre to the circumference they grow continually rarer and rarer, till at the circumference they become infinitely rare: for which reason, these dispersed rays are not copious enough to be visible, except about the centre of the circle. He also mentions another argument to prove, that the different refrangibility of the rays of light is the true cause of the imperfection of Telescopes. For the dispersions of the rays arising from the spherical figures of objectglasses, are as the cubes of their apertures; and therefore, to cause Telescopes of different lengths to magnify with equal distinctness, the apertures of the objectglasses, and the charges or magnifying powers ought to be as the cubes of the square roots of their lengths, which does not answer to experience. But the errors of the rays, arising from the different refrangibility, are as the apertures of the object-glasses; and thence, to make Telescopes of different lengths to magnify with equal distinctness, their apertures and charges ought to be as the square roots of their lengths; and this answers to experience.
Were it not for this different refrangibility of the rays, Telescopes might be brought to a sufficient degree of perfection, by composing the object-glass of two glasses with water between them. For by this means, the refractions on the concave sides of the glasses will very much correct the errors of the refractions on the convex sides, so far as they arise from their spherical figure: but on account of the different refrangibility of different kinds of rays, Newton did not see any other means of improving Telescopes by refraction only, except by increasing their length. Newton's Optics, pa. 73, 83, 89, 3d edition.
This important desideratum in the construction of dioptric Telescopes, has been since discovered by the ingenious Mr. Dollond; an account of which is given below.
Achromatic Telescope, is a name given to the refracting Telescope, invented by Mr. John Dollond, and so contrived as to remedy the aberration arising from colours, or the different refrangibility of the rays of light. See Achromatic.
The principles of Mr. Dollond's discovery and construction, have been already explained under the articles Aberration, and Achromatic. The improvement made by Mr. Dollond in his Telescopes, by making two object-glasses of crown-glass, and one of flint, which was tried with success when concave eye-glasses were used, was completed by his son Peter Dollond; who, conceiving that the same method might be practised with success with convex eye-glasses, found, after a few trials, that it might be done. Accordingly he finished an object-glass of 5 feet focal length, with an aperture of 3 3/4 inches, composed of two convex lenses of crown-glass, and one concave of white flint glass. But apprehending afterward that the apertures might be admitted still larger, he completed one of 3 1/2 feet focal length, with the same aperture of 3 3/4 inches. Philos. Trans. vol. 55, p. 56.
But beside the obligation we are under to Mr. Dollond, 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 in Telescopes, viz, by correcting, in a great measure, 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 degrees, the extreme pencils of the field must be bent or refracted in an angle of 10 degrees; which, if it be performed by one eye-glass, will cause an aberration from the figure, in proportion to the cube of that angle: but if two glasses be 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 this case proportional to double the cube of half the angle, will be but a 4th part of that which is in proportion to the cube of the whole angle; because twice the cube of 1 is but 1/4 of the cube of 2: so that the aberration from the figure, where two eye-glasses are rightly proportioned, is but a 4th part of what it must unavoidably be, where the whole is performed by a single eye-glass. By the same way of reasoning, when the refraction is divided among three glasses, the aberration will be found to be but the 9th part of what would be produced from a single glass; because 3 times the cube of 1 is but the 9th part of the cube of 3. Whence it appears, that by increasing the number of eye-glasses, the indistinctness, near the borders of the field of a Telescope, may be very much diminished, though not entirely taken away.
The method of correcting the errors arising from the different refrangibility of light, is of a different consideration from the former: for, whereas the errors from the figure can only be diminished in a certain proportion 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 colours quite to the borders of the field. Also, in the day telescope, where no more than two eye-glasses are absolutely necessary for erecting the object, we find, by the addition of a third rightly situated, that the colours, which would otherwise confuse the image, are entirely removed: but this must be understood with some limitation; for though the different colours, which the extreme pencils must necessarily be divided into by the edges of the eye-glasses, may in this manner be brought to the eye in a direction parallel to each other, so as, by its humours, to be converged to a point in the retina, yet if the glasses exceed a certain length, the colours may be spread too wide to be capable of being admitted through the pupil or aperture of the eye; which is the reason that, in long Telescopes, constructed in the common way, with three eye-glasses, the field is always very much contracted.
These considerations first set Mr. Dollond upon contriving how to enlarge the field, by increasing the number of eye-glasses, without any hindrance to the distinctness or brightness of the image: and though others had been about the same work before, yet observing that the five-glass Telescopes, sold in the shops, would admit of farther improvement, he endeavoured to construct one with the same number of glasses in a better manner; which so far answered his expectations, as to be allowed by the best judges to be a considerable improvement on the former. Encouraged by this success, he resolved to try if he could not make some farther enlargement of the field, by the addition of another glass, and by placing and proportioning the glasses in such a manner, as to correct the aberrations as much as possible, without any detriment to the distinctness: and at last he obtained as large a field as is convenient or necessary, and that even in the longest Telescopes that can be made. These Telescopes, with 6 glasses, having been well received both at home and abroad, the author has settled the date of the invention in a letter addressed to Mr. Short, and read at the Royal Society, March 1, 1753. Philos. Trans. vol. 48, art. 14.
Of the Achromatic Telescopes, invented by Mr. Dollond, there are several different sizes, from one foot to 8 feet in length, made and sold by his sons P. and J. Dollond. In the 17 - inch improved Achromatic Telescope, the object glass is composed of three glasses, viz, two convex of crown-glass, and one concave of white flint-glass: the focal distance of this combined object-glass is about 17 inches, and the diameter of the aperture 2 inches. There are 4 eye-glasses contained in the tube, to be used for land objects; the magnifying power with these is near 50 times; and they are adjusted to different sights, and to different distances of the object, by turning a finger screw at the end of the outer tube. There is another tube, containing two eye-glasses that magnify about 70 times, for astronomical purposes. The Telescope may be directed to any object by turning two screws in the stand on which it is sixed, the one giving a vertical motion, and the other a horizontal oue. The stand may be inclosed in the inside of the brass tube.
The object-glass of the 2 1/2 and 3 1/2 feet Telescopes is composed of two glasses, one convex of crown glass, and the other concave of white flint glass; and the diameters of their apertures are 2 inches and 2 3/4 inches. Each of them is furnished with two tubes; one for land objects, containing four eye-glasses, and another with two eye-glasses for astronomical uses. They are adjusted by buttons on the outside of the wooden tube; and the vertical and horizontal motions are given by joints in the stands. The magnifying power of the least of these Telescopes, with the eye-glass for land objects, is near 50 times, and with those for astronomical purposes, 80 times; and that of the greatest for land objects is near 70 times, but for astronomical observations 80 and 130 times; for this has two tubes, either of which may be used as occasion requires. This Telescope is also moved by a screw and rackwork, and the screw is turned by means of a Hook's joint.
These opticians also construct an Achromatic pocket perspective glass, or Galilean Telescope; so contrived, that all the different parts are put together and contained in one piece 4 1/2 inches long. This small Telescope is furnished with 4 concave eye-glasses, the magnifying powers of which are 6, 12, 18, and 28 times. With the greatest power of this Telescope, the satellites of Jupiter and the ring of Saturn may be easily seen. They have also contrived an Achromatic Telescope, the sliding tubes of which are made of very thin brass, which pass through springs or tubes; the outside tube being either of mahogany or brass. These Telescopes, which from their convenience for gentlemen in the army are called military Telescopes, have 4 convex eye- | glasses, whose surfaces and focal lengths are so proportioned, as to render the field of view very large. They are of 4 different lengths and sizes, usually called one foot, 2, 3, and 4 feet: the first is 14 inches when in use, and 5 inches when shut up, having the aperture of the object-glass 1 1/10 inch, and magnifying 22 times: the second 28 inches for use, 9 inches shut up, the aperture 1 6/10 inch, and magnifying 35 times; the third 40 inches, and 10 inches shut, with the aperture 2 inches, and magnifying 45 times; and the sourth 52 inches, and 14 inches shut, with the aperture 2 3/4 inches, and magnifying 55 times.
Mr. Euler, who, in a memoir of the Academy of Berlin for the year 1757, p. 323, had calculated the effects of all possible combinations of lenses in Telescopes and microscopes, published another long memoir on the subject of these Telescopes, shewing with precision of what advantages they are naturally capable. See Miscel. Taurin. vol. 3, par. 2, pag. 92.
Mr. Caleb Smith, having paid much attention to the subject of shortening and improving Telescopes, thought he had found it possible to rectify the errors which arise from the different degrees of refrangibility, on the principle that the sines of refraction of rays differently refrangible, are to one another in a given proportion, when their sines of incidence are equal; and the method he proposed for this purpose, was to make the specula of glass, instead of metal, the two surfaces having different degrees of concavity. But it does not appear that this scheme was ever carried into practice. See Philos. Trans. number 456, pa. 326, or Abr. vol. 8, pa. 113.
The ingenious Mr. Ramsden has lately described a new construction of eye-glasses for such Telescopes as may be applied to mathematical instruments. The construction which he proposes, is that of two planoconvex lenses, both of them placed between the eye and the observed image formed by the object-glass of the instrument, and thus correcting not only the aberration arising from the spherical figure of the lenses, but also that arising from the different refrangibility of light. For a more particular account of this construction, its principle, and its effects, see Philos. Trans. vol. 73, art. 5.
A construction, similar at least in its principle to that above, is ascribed, in the Synopsis Optica Honorati Fabri, to Eustachio Divini, who placed two equal narrow plano-convex lenses, instead of one eye lens, to his Telescopes, which touched at their vertices; the focus of the object-glass coinciding with the centre of the plano-convex lens next it. And this, it is said, was done at once both to make the rays that come parallel from the object fall parallel upon the eye, to exclude the colours of the rainbow from it, to augment the angle of sight, the field of view, the brightness of the object, &c. This was also known to Huygens, who sometimes made use of the same construction, and gives the theory of it in his Dioptrics. See Hugenii Opera Varia, vol. 4, ed. 1728.
Telescope, Reflecting, or Catoptric, or Catadioptric, is a Telescope which, instead of lenses, consists chiefly of mirrors, and exhibits remote objects by reflection instead of refraction.
A brief account of the history of the invention of this important and useful Telescope, is as follows. The ingenious Mr. James Gregory, of Aberdeen, has been commonly considered as the first inventor of this Telescope—But it seems the first thought of a reflector had been suggested by Mersenne, about 20 years before the date of Gregory's invention: a hint to this purpose occurs in the 7th proposition of his Catoptrics, which was printed in 1651: and it appears from the 3d and 29th letters of Descartes, in vol. 2 of his Letters, which it is said were written in 1639, though they were not published till the year 1666, that Mersenne proposed a Telescope with specula to Descartes in that correspondence; 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 convince Mersenne of its fallacy. This point has been largely discussed by Le Roi in the Encyclopedia, art. Telescope, and by Montucla in his Hist. des Mathem. tom. 2, p. 643.
Whether Gregory had seen Mersenne's treatise on optics and catoptrics, and whether he availed himself of the hint there suggested, or not, perhaps cannot now be determined. He was led however to the inventi<*> by seeking to correct two imperfections in the common Telescope: the first of these was its too great length, which made it troublesome to manage; and the second was the incorrectness of the image. It had been already demonstrated, that a pencil of rays could not be collected in a single point by a spherical lens; and also, that the image transmitted by such a lens would be in some degree incurvated. These inconveniences he thought might be obviated by substituting for the object-glass a metallic speculum, of a parabolical 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 was, for that purpose, to be perforated in its centre. This construction he published in 1663, in his Optica Promota. But as Gregory, according to his own account, possessed no mechanical skill, and could not find a workman capable of realizing his invention, after some fruitless trials, he was obliged to give up the thoughts of bringing Telescopes of this kind into use.
Sir Isaac Newton however interposed, to save this excellent invention from perishing, and to bring it forward to maturity. Having applied himself to the improvement of the Telescope, and 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 formed from one of the conic sections. But, in the year 1666, having discovered the different refrangibility of the rays of light, and finding that the errors of Telescopes, arising from that cause alone, were much more considerable than such as were occasioned by the spherical figure of lenses, he was constrained to turn his thoughts to reflectors. The plague however interrupted his progress in this business; so that it was towards the end of 1668, or in the beginning of 1669, when, despairing of perfecting Telescopes by means of refracted light, | and recurring to the construction of reflectors, he set about making his own specula, and early in the year 1672 completed two small reflecting Telescopes. In these he ground the large speculum into a spherical concave, being unable to accomplish the parabolic form proposed by Gregory; but though he then despaired of performing that work by geometrical rules, yet (as he writes in a letter that accompanied one of these instruments, which he presented to the Royal Society) he doubted not but that the thing might in some measure be accomplished by mechanical devices. With a perseverance equal to his ingenuity, he, in a great measure, overcame another difficulty, which was to find a metallic substance that would be of a proper hardness, have the fewest pores, and receive the smoothest polish: this difficulty he deemed almost insurmountable, when he considered that every irregularity in a reflecting surface would make the rays of light deviate 5 or 6 times more out of their due course, than the like irregularities in a refracting surface. After repeated trials, he at last found a composition that answered in some degree, leaving it to those who should come after him to find a better. These difficulties have accordingly been since obviated by other artists, particularly by Dr. Mudge, the rev. Mr. Edwards, and Dr. Herschel, &c. Newton having succeeded so far, he communicated to the Royal Society a full and satisfactory account of the construction and performance of his Telescope. The Society, by their secretary Mr. Oldenburgh, transmitted an account of the discovery to Mr. Huygens, celebrated as a distinguished improver of the refractor; who not only replied to the Society in terms expressing his high approbation of the invention, but drew up a favourable account of the new Telescope, which he caused to be published in the Journal des Sçavans of the year 1672, and by this mode of communication it was soon known over Europe. See Huygenii Opera Varia, tom. 4.
Notwithstanding the excellence and utility of this contrivance, and the honourable manner in which it was announced to the world, it seems to have been greatly neglected for nearly half a century. Indeed when Newton had published an account of his Telescopes in the Philos Trans. M. Cassegrain, a Frenchman, in the Journal des Sçavans of 1672, claimed the honour of a similar invention, and said, that, before he heard of Newton's improvement, he had hit upon a better construction, by using a small convex mirror instead of the reflecting prism. This Telescope, which was the Gregorian one disguised, the large mirror being perforated, and which it is said was never executed by the author, is much shorter than the Newtonian; and the convex mirror, by dispersing the rays, serves greatly to increase the image made by the large concave mirror.
Newton made many objections to Cassegrain's construction, but several of them equally affect that of Gregory, which has been found to answer remarkably well in the hands of good artists.
Dr. Smith took the pains to make many calculations of the magnifying power, both of Newton's and Cassegrain's Telescopes, in order to their farther improvement, which may be seen in his Optics, Rem. p. 97.
Mr. Short, it is also said, made several Telescopes on the plan of Cassegrain.
Dr. Hook constructed a Reflecting Telescope (mentioned by Dr. Birch in his Hist. of the Royal Soc. vol. 3, p. 122) in which the great mirror was perforated, so that the spectator looked directly towards the object, and it was produced before the Royal Society in 1674. On this occasion it was said that this construction was first proposed by Mersenne, and afterwards repeated by Gregory, but that it never had been actually executed before it was done by Hook. A description of this instrument may be seen in Hook's Experiments, by Derham, p. 269.
The Society also made an unsuccessful attempt, by employing an artisicer to imitate the Newtonian construction; however, about half a century after the invention of Newton, a Reflecting Telescope was produced to the world, of the Newtonian construction, which the venerable author, ere yet he had finished his very distinguished course, had the satisfaction to find executed in such a manner, as left no room to fear that the invention would longer continue in obscurity. This effectual service to science was accomplished by Mr. John Hadley, who, in the year 1723, presented to the Royal Society a Telescope, which he had constructed upon Newton's plan. The two Telescopes which Newton had made, were but 6 inches long, were held in the hand for viewing objects, and in power were compared to a 6-feet refractor: but the radius of the sphere, to which the principal speculum of Hadley's was ground, was 10 feet 5 1/4 inches, and consequently its focal length was 62 5/8 inches. In the Philos. Trans. Abr. vol. 6, p. 133, may be seen a drawing and description of this Telescope, and also of a very ingenious but complex apparatus, by which it was managed. One of these Telescopes, in which the focal length of the large mirror was not quite 5 1/4 feet, was compared with the celebrated Huygenian Telescope, which had the focal length of its object-glass 123 feet; and it was found that the former would bear such a charge, as to make it magnify the object as many times as the latter with its due charge; and that it represented objects as distinctly, though not altogether so clear and bright. With this Reflecting Telescope might be seen whatever had been hitherto discovered by the Huygenian, particularly the transits of Jupiter's satellites, and their shades over the disk of Jupiter, the black list in Saturn's ring, and the edge of the shade of Saturn cast upon his ring. Five satellites of Saturn were also observed with this Telescope, and it afforded other observations on Jupiter and Saturn, which confirmed the good opinion which had been conceived of it by Pound and Bradley.
Mr. Hadley, after finishing two Telescopes of the Newtonian construction, applied himself to make them in the Gregorian form, in which the large mirror is perforated. This scheme he completed in the year 1726.
Dr. Smith prefers the Newtonian construction to that of Gregory; but if long experience be admitted as a final judge in such matters, the superiority must be adjudged to the latter; as it is now, and has been for many years past, the only instrument in request. |
Mr. Hadley spared no pains, after having completed his construction, to instruct Mr. Molyneux and Dr. Bradley; and when these gentlemen had made a good proficiency in the art, being desirous that these Telescopes should become more public, they liberally communicated to some of the chief instrument makers of London, the knowledge they had acquired from him: and thus, as it is reasonable to imagine, reflectors were completed by other and better methods than even those in which they had been instructed. Mr. James Short in particular signalized himself as early as the year 1734, by his work in this way. He at first made his specula of glass; but finding that the light reflected from the best glass specula was much less than the light reflected from metallic ones, and that glass was very liable to change its form by its own weight, he applied himself to improve metallic specula; and, by giving particular attention to the curvature of them, he was able to give them greater apertures than other workmen could do; and by a more accurate adjustment of the specula, &c, he greatly improved the whole instrument. By some which he made, in which the larger mirror was 15 inches focal distance, he and some other persons were able to read in the Philos. Trans. at the distance of 500 feet; and they several times saw the five satellites of Saturn together, which greatly surprised Mr. Maclaurin, who gave this account of it, till he found that Cassini had sometimes seen them all with a 17 feet refractor. Short's Telescopes were all of the Gregorian construction. It is supposed that he discovered a method of giving the parabolic figure to his great speculum; a degree of perfection which Gregory and Newton despaired of attaining, and which Hadley it seems had never attempted in either of his Telescopes. However, the secret of working that configuration, whatever it was, it seems died with that ingenious artist. Though lately in some degree discovered by Dr. Mudge and others.
On the History of Reflecting Telescopes, see Dr. David Gregory's Elem. of Catopt. and Dioptr. Appendix by Desaguliers: Smith's Optics, book 3, c. 2, Rem. on art. 489: and Sir John Pringle's excellent Discourse on the Invention &c of the Reflecting Telescope.
Construction of the Reflecting Telescope of the Newtonian form.—Let ABCD (fig. 2, pl. 32) be a large tube, open at AD, and closed at BC, and its length at least equal to the distance of the focus from the metallic spherical concave speculum GH placed at the end BC. The rays EG, FH, &c, proceeding from a remote object PR, intersect one another somewhere before they enter the tube, so that EG and eg are those that come from the lower part of the object, and fh FH from its upper part: these rays, after falling on the speculum GH, will be reflected so as to converge and meet in mn, where they will form a perfect image of the object. But as this image cannot be seen by the spectator, they are intercepted by a small plane metallic speculum KK, intersecting the axis at an angle of 45°, by which the rays tending to m, n, will be reflected towards a hole LL in the side of the tube, and the image of the object will be thus formed in qS; which image will be less distinct, because some of the rays which would otherwise fall on the concave speculum GH, are intercepted by the plane speculum: it will nevertheless appear pretty distinct, because the aperture AD of the tube, and the speculum GH, are large. In the lateral hole LL is fixed a convex lens, whose focus is at Sq; and therefore this lens will refract the rays that proceed from any point of the image, so as at their exit they will appear parallel, and those that proceed from the extreme points S, q, will converge after refraction, and form an angle at O, where the eye is placed; which will see the image Sq, as if it were an object, through the lens LL: consequently the object will appear enlarged, inverted, bright, and distinct. In LL may be placed lenses of different convexities, which, by being moved nearer to the image and farther from it, will represent the object more or less magnified, if the surface of the speculum GH be of a figure truly spherical. If, instead of one lens LL, three lenses be disposed in the same manner with the three eye-glasses of the refracting Telescope, the object will appear erect, but less distinct than when it is observed with one lens. On account of the position of the eye in this Telescope, it is extremely difficult to direct the instrument towards any object: Huygens therefore first thought of adding to it a small refracting Telescope, having its axis parallel to that of the reflector: this is called a finder or director. The Newtonian Telescope is also furnished with a suitable apparatus for the commodious use of it.
To determine the magnifying power of this Telescope, it is to be considered that the plane speculum KK is of no use in this respect: let us then suppose that one ray proceeding from the object coincides with the axis GLIA of the lens and speculum; let bb be another ray proceeding from the lower extremity of the object, and passing through the focus I of the speculum KH; this will be reflected in the direction bid, parallel to the axis GLA, and falling on the lens dLd, will be refracted to G, so that GL will be equal to LI, and dG = dI. To the naked eye the object would appear under the angle Ibi = bIA; but by means of the Telescope it appears under the angle dGL = dIL = Idi: and the angle Idi is to the angle Ibi as bI to Id; consequently the apparent magnitude by the Telescope, is to that with the naked eye, as the distance of the focus of the speculum from the speculum, to the distance of the focus of the lens from the lens.
Construction of the Gregorian Reflecting Telescope.— Let TYYT (fig. 3, pl. 32) be a brass tube, in which EldD is a metallic concave speculum, perforated in the middle at X; and EF a less concave mirror, so sixed by the arm or strong wire RT, which is moveable by means of a long screw on the outside of the tube, as to be moved nearer to, or farther from the larger speculum LldD; its axis being kept in the same line with that of the great one. Let AB represent a very remote object, from each part of which issue pencils of rays, as cd, CD, from A the upper extremity of the | object, and IL, il, from the lower part B; the rays IL, CD, from the extremities, crossing one another before they enter the tube. These rays, falling upon the larger mirror LD, are reflected from it into the focus KH, where they form an inverted image of the object AB, as in the Newtonian Telescope. From this image the rays, issuing as from an object, fall upon the small mirror EF, the centre of which is at e, so that after reflection they would meet in their foci at QQ, and there form an erect image. But since an eye at that place could see but a small part of an object, in order to bring rays from more distant parts of it into the pupil, they are intercepted by the plano-convex lens MN, by which means a smaller erect image is formed at PV, which is viewed through the meniscus SS, by an eye at O. This meniscus both makes the rays of each pencil parallel, and magnifies the image PV. At the place of this image all the foreign rays are intercepted by the perforated partition ZZ. For the same reason the hole near the eye O is very narrow. When nearer objects are viewed by this Telescope, the small speculum EF is removed to a greater distance from the larger LD, so that the second image may be always formed in PV: and this distance is to be adjusted (by means of the screw on the outside of the great tube) according to the form of the eye of the spectator. It is also necessary that the axis of the Telescope should pass through the middle of the speculum EF, and its centre, the centre of the speculum LL, and the middle of the hole X, the centres of the lenses MN, SS, and the hole near O. As the hole X in the speculum LL can reflect none of the rays issuing from the object, that part of the image which corresponds to the middle of the object, must appear to the observer more dark and confused than the extreme parts of it. Besides, the speculum EF will also intercept many rays proceeding from the object; and therefore, unless the aperture TT be large, the object must appear in some degree obscure.
The magnifying power of this Telescope is estimated in the following manner. Let LD be the larger mirror (fig. 3, pl. 31), having its focus at G, and aperture in A; and FF the small mirror with the focus of parallel rays in I, and the axis of both the specula and lenses MN, SS, be in the right line DIGAOK. Let bb be a ray of light coming from the lower extremity of a very distant visible object, passing through the focus G, and falling upon the point b of the speculum LD; which, after being reflected from b to F in a direction parallel to the axis of the mirror DAK, is reflected by the speculum F so as to pass through the focus I in the direction FIN to N, at the extremity of the lens MN, by which it would have been refracted to K; but by the interposition of another lens SS is brought to O, so that the eye in O sees half the object under the angle TOS. The angle GbF, or AGb, under which the object is viewed by the naked eye, is to SOT under which it is viewed by the Telescope, in the ratio of GbF to IFi = nIN, of nIN to NKn, and of NKn to SOT.
In Reflecting Telescopes of different lengths, a given object will appear equally bright and equally distinct, when their linear apertures, and also their linear breadths, are as the 4th roots of the cubes of their lengths; and consequently when the focal distances of their eyeglasses are also as the 4th roots of their lengths. See the demonstration of this proposition in Smith's Optics, art. 361.
Hence he has deduced a rule, by which he has computed the following table for Telescopes of different lengths, taking, for a standard, the middle eye-glass and aperture of Hadley's Reflecting Telescope, described in Philos. Trans. number 376 and 378: the focal distances and linear apertures being given in 1000th parts of an inch.
| Table for Telescopes of different Lengths. | |||
| Length of | Focal dist. | Linear am- | Linear a- |
| the Tel. or | plifying or | perture of | |
| focal dist. of | of the | magnifying | the concave |
| the cone. | Eye-glass. | power. | metal. |
| feet | inches | --- | inches |
| 1/2 | 0.167 | 36 | 0.864 |
| 1 | 0.199 | 60 | 1.440 |
| 2 | 0.236 | 102 | 2.448 |
| 3 | 0.261 | 138 | 3.312 |
| 4 | 0.281 | 171 | 4.104 |
| 5 | 0.297 | 202 | 4.843 |
| 6 | 0.311 | 232 | 5.568 |
| 7 | 0.323 | 260 | 6.240 |
| 8 | 0.334 | 287 | 6.888 |
| 9 | 0.344 | 314 | 7.536 |
| 10 | 0.353 | 340 | 8.160 |
| 11 | 0.362 | 365 | 8.760 |
| 12 | 0.367 | 390 | 9.360 |
| 13 | 0.377 | 414 | 9.936 |
| 14 | 0.384 | 437 | 10.488 |
| 15 | 0.391 | 460 | 11.040 |
| 16 | 0.397 | 483 | 11.592 |
| 17 | 0.403 | 506 | 12.143 |
Mr. Hadley's Telescope, above mentioned, magnified 228 or 230 times; but we are informed that an object-metal of 3 1/4 feet focal distance was wrought by Mr. Hauksbee to so great a perfection, as to magnify 226 times, and therefore it was scarcely inferior to Hadley's of 5 1/2 feet. If Hauksbee's Telescope be taken for a new standard, it follows that a speculum of one foot focal distance ought to magnify 93 times, whereas the above table allows it but 60. Now 93/60 = 1.55, and the given column of magnifying powers multiplied by this number, gives a new column, shewing how much the object-metals ought to magnify if wrought up to the perfection of Hauksbee's. And thus a new table may be easily made for this or any other more perfect standard, taking also the new eye-glasses and apertures in the same ratio to one another as the old ones have in this table. Smith's Optics, Rem. p. 79. |
The magnifying power of any Telescope may be easily found by experiment, viz, by looking with one eye through the Telescope upon an object of known dimensions, and at a given distance, and throwing the image upon another object seen with the naked eye. Dr. Smith has given a particular account of the process, Rem. p. 79.
But the easiest method of all, is to measure the diameter of the aperture of the object-glass, and that of the little image of it, which is formed at the place of the eye. For the proportion between these gives the ratio of the magnifying power, provided no part of the original pencil be intercepted by the bad construction of the Telescope. For in all cases the magnifying power of Telescopes, or microscopes, is measured by the proportion of the diameter of the original pencil, to that of the pencil which enters the eye. Priestley's Hist. of Light, p. 747.
But the most considerable, and indeed truly astonishing magnifying powers, that have ever been used, are those of Dr. Herschel's Reflecting Telescopes. Some account of these, and of the discoveries made by them, has been already introduced under the article Star. For his method of ascertaining them, see Philos. Trans. vol. 72, pa. 173 &c. See also several of the other late volumes of the Philos. Trans.
Dr. Herschel observes, that though opticians have proved, that two eye-glasses will give a more correct image than one, he has always (from experience) persisted in refusing the assistance of a second glass, which is sure to introduce errors greater than those he would correct. “Let us resign, says he, the double eyeglass to those who view objects merely for entertainment, and who must have an exorbitant field of view. To a philosopher, this is an unpardonable indulgence. I have tried both the single and double eye-glass of equal powers, and always found that the single eye-glass had much the superiority in point of light and distinctness. With the double eye-glass I could not see the belts in Saturn, which I very plainly saw with the single one. I would however except all those cases where a large field is absolutely necessary, and where power joined to distinctness is not the sole object of our view.” Philos. Trans. vol. 72, p. 95.
Mr. Green of Deptford has lately added both to the reflecting and refracting Telescope an apparatus, which fits it for the purposes of surveying, levelling, measuring angles and distances, &c. See his Description and Use of the improved Reflecting and Refracting Telescopes, and Scale of Surveying &c, 1778.— Mr. Ramsden too has lately adapted Telescopes to the like purpose of measuring distances from one station, &c.
Meridian Telescope, is one that is fixed at right angles to an axis, and turned about it in the plane of the meridian; and is otherwise called a transit instrument.—The common use of it is to correct the motion of a clock or watch, by daily observing the exact time when the sun or a star comes to the meridian. It serves also for a variety of other uses. The transverse axis is placed horizontal by a spirit level. For the farther description and method of fixing this instrument by means of its levels &c, see Smith's Optics, p. 321. See also Transit Instrument.
TELESCOPICAL Stars, are such as are not visible to the naked eye, being only discernible by means of a telescope. See Star.
All stars less than those of the 6th magnitude, are Telescopic to an ordinary eye.
