TEL 



TEL 



ne perceived a small object to be magnified when \ ic , ,1 

 through a convex U-ns. 



It is hisrlily probable that tin- telescope had been in- 

 vented lui. - lu- value t>i' MU-II an instrument was 

 duly appreciated: mill it may have been owing to the 

 very irradual discovery of its importance that the name 

 I' the inventor sunk into oblivion : about the middle 

 of the seventeenth cent urv. houcver.au effort was made 

 to discover the trace* of UM invention, anil 1'ctcr Horellus, 

 in his work entitled ' l)e vero Tclcscopii Invcntorc,' which 

 was publislied in MJ.VI at the Hague, lias iriven te-timo- 

 nials in favour of two persons; the first of these is Xacha- 

 riah Jans, or Jansen, ami the other, Hans Lapprev. or Lip- 

 persheiin. both of whom are said to have been opticians, 

 or spectacle-makers, residing at Middleburgh : in B letter 

 written by a son of .Jans, n is Mated that the epoch of the 

 discovery is the year 15SK); but by another account, the 

 v ear IC10. The same author has also given a letter from 

 M. William Boreel (envoy from the States of Holland to 

 the British Court) which seems to throw some light on the 

 facts. The writer of the letter asserts that he was ac- 

 quainted with the younger Zachariah Jans, when both of 

 tnem were children, and had often heard that the elder was 

 the inventor of the nnrniK'-,,/,,' . I,,- adds that, about the year 

 1610, the two opticians Jans and l.apprey first const meted 

 telescopes, and that they presented one to Prince Maurice 

 of Nassau, who desired that the invention might be kept 

 secret as (the United Provinces being then at war with 

 France) he expected to obtain in the field, by means oi 

 the instalment, some advantages over the enemy. The 

 writer further states that the invention became known, 

 and that soon afterwards Adrian Metius and Cornelius 

 Drebbel went to Middleburgh and purchased telescopes 

 at the house of Jans. This account (litters from that 

 which is given by Descartes (' Dioptrics,' cap. 1 , who. 

 writing in Holland, states that about thirty years pre- 

 viously, Metins (who was, he observes, a native of Alck- 

 maer)l having always taken pleasure in forming burning- 

 mirrors and lenses of glass and of ice, by chance placed 

 at the extremities of a tube two lenses, one thicker in tin 

 middle, and the other thinner, than about the edge (con- 

 vex and concave) ; and thus, he adds, was formed the in- 

 strument which is called a telescope. The 'Dioptrics' was 

 published at Leyden in 1G37, and therefore the time ol 

 the supposed invention by Metius is nearly coincident 

 with that at which, according to Horellus. it \V:LS made b\ 

 Jans. From the papers of Harriot, it appears that this 

 m.Ulu matician observed spots on the sun, in 1610, will 

 clcscopcs magnifying from 10 to 30 times ; but it is 

 uncertain whether he got them from Holland, or whether 

 they were made in this country ; and the only conclusions 

 at which it is possible now to arrive, are, that telescopes 

 were known in England and Holland about the end of the 

 sixteenth century, and thai in both countries they were 

 then in a form which rendered them practically useful. 



The two .Ian-ens, father and son, appear to have usec 

 their telescopes in observing the heavens ; and the lattei 

 is said to have remarked four small stars near Jupiter: i 

 has been concluded from thence, that he was the first dis 

 coverer of the satellites of that planet; but though thii 

 may be, he probably did not continue his observation- 

 long enough to enable him to determine their distances 

 from it, or the times of their revolutions. 



The use of the telescope, and, probably, even the know 

 ledge of the fact that it had been invented, mu-t havi 

 : for many years confined to the north of Kurope : to 

 it appears that it was not till the year HXK) that (lalilen 

 who then happened to be at Venice, heard from a German 

 a rumour of the discovery which was said to have beei 

 made in Holland. The Italian philosopher states, in tin 

 Sidereus Nuneius,' that he had then no knowledge o 

 the nature of the instrument, and that he requested a 

 friend at Pan* to send him some information concerning 

 it. On being informed, merely, that it was a tube con- 

 taining glass lenses, his acquaintance with the nature 01 

 the refraction of light enabled him, it is said, to discover 

 that one of the lenses must have been convex and the 

 other concave, and also to determine the distance at 

 which they should be placed from one another in order 

 that the objects seen through them might appear magni- 

 fied and distinct. Without however supposing that 

 Galileo was here guturd by theoretical considerations 

 merely, it is easy to conceive that, as lenses of different 



.mil- were then in use for spectacles, ho miu-ht have ob- 

 ained from an optician some which were nl different 

 legrees of convexity and concavity : and aller n few trials 

 ^le must have found such as would constitute an instru- 

 uent possessing magnifying pov- 



The telescopes which he constructed consisted of one 

 onvex object-glass and one concave eye-glass, whieh 

 were placed at the extremities of a leaden tube ; and the 

 "irst of them magnified the heights and breadths of objects 

 three times only. Soon afterwards he made one which 

 magnified eiirht times; and subsequently he succeeded 

 in funning a telescope with a magnifying power which 

 caused objects to appear about thirty times as great as 

 they are to the unassisted eye. 



The knowledge whieh man had acquired of the visible 

 heavens received many important accessions from thi 

 coveries which Galileo was enabled to make by mea. 

 the telescope. Except the sun and moon, not one of tin- 

 celestial bodies had hitherto been observed to have any 

 visible form or magnitude, and it was to the eye of rt 

 alone that those appeared to be anything but plane sur- 

 faees : the fixed stars and the planets were alike known 

 only ns luminous and ill-defined points: but when 

 through a telescope, the planets were found to ha\. 

 tain magnitudes, and some of them to undergo variations 

 of form ; while the fixed stars appeared unchanged, or 

 only divested of the radiance with which they seem to be 

 surrounded when seen by the naked eye : and hence it 

 became obvious that the former must constitute a distinct 

 group of bodies infinitely nearer the earth than the others. 

 The sun, from the spots observed on his surface, was found 

 to revolve on its axis, and consequently was ascertained 

 to be globular; and the light and dark spaces on tin- 

 moon were distinctly perceived to be mountains and val- 

 Icvs. nearly resembling those features on the surface of tin- 

 earth. Galileo relates, in the work above mentioned, that 

 in the year 1010 he discovered the four satellites of Jupi- 

 ter, and observed that they revolved about that planet a-. 

 our moon revolves about the earth. Nearly at the 

 time he observed that Saturn presented a remarkable ap- 

 pearance : at first he thought it was accompanied by two 

 smaller planets; but on using a telescope of superior 

 magnifying power, these were found to be portions of a 

 vast annulus which surrounds Saturn without touehini; 

 his surface; and soon afterwards he ascertained the fact 

 that Venus exhibited phases similar to those of the 

 moon. 



The species of telescope which was used by Galileo- 

 continued for several years unchanged : yet it is ex- 

 tremely defective, on account of the small extent of the 

 field of view which it affords when its magnifying ]> 

 is considerable ; and the Hatavian or Galilean telescope-, 

 as it was called, is now chiefly used to distinguish object-- 

 in a theatre. It is due to the memory of Kepler to state 

 that he pointed out (in his ' Dioptrics' > the possibility of 

 forming telescopes with two lenses, both of whicU nix- 

 convex; but he did not reduce his ideas to practice by the- 

 construction of such an Instrument, and the honour of 

 having been the first to do so is to be attributed to the 

 Jesuit Seheiner. who, in his Kosa Ursina ' (1CTXI , i_ r i\c- 

 a description of telescopes with one convex eye-glass. 

 He observes that the the imaircs of objects to 



appear in inverted positions; and adds, that thirteen years 

 previously he had used such a telescope in prescni 

 the Archduke Maximilian. 



Tele-copes with a sinerle convex eye-glass have 1 

 since designated astronomical, from the circumstance that 

 they were long employed for celestial observations; the 

 r extent of their field of view having caused them. 

 notwithstanding the inversion of the image, to SH|M 

 for that purpose the telescopes of Galileo. It ought to 

 be remarked however that telescopes with two <-, 

 by which the object might be seen in a dii n, as 



it appears to the naked eye. were described by Kepler, 

 and constructed by Seheiner; but as they caused the 

 object to appear much distorted and coloured abnut the 

 margin of the field, they were not esteemed. Pcre de 

 Kheita, about the same time, constructed for telescopes. 

 eve-tubes containing three lenses, which, he observe-. 

 afford a better image than those with two : the same per- 

 son was the inventor of what is called a binocular tele- 

 scope, that is, an instrument which consists of two tele- 

 scope* haying equal magnifying powers, and placed near 



