TEL 



1G6 



T E L 



v. '.. ! telescopes, both o. ' mian and 







i!ly 1u tli In* )>liuit>t which in called 



i' formation of a speculum lour 

 feet in diamet. T and in focal 1, 



to which it iippertai ;i kind, tin.' 



i-ing placed in : pen mil oi ihc 



tulte, nnd viewing the imase thro . 



\Vith this ' . <<!' in 



moznitied nl'out (if**) time*: and on the 

 night alter it v, i. Dr. Hi il thu 



sixth satellite, of Saturn. An ntlrm]>t is even now being 

 to form a reflect in-,' telescope possessing a higher 

 ion than that of Hemchel; but thuirh 

 the expectation should be fulfilled, telescopes of groat 

 magnitude must always be inconvenient in the in: 

 incut ; and. from 1hr'e\pen-e of their construction, they 

 will ever In' confined to a few persons. It is right to ob- 

 serve moreover that the greatest discoveries of which 

 astronomy cnn boast have been made with telescopes 

 w hose macmfving power did not exceed 700 times. 



While the unproTement of reflecting: b- win 



progress, the efforts to combine glass lenses in order to 

 dimmish the colouri'd fringes by which the images in diop- 

 trical telescopes are surrounded were not entirely neg- 

 lected : and as early as 172!), a private gentleman. Mr. 

 ion.' Hall, of Essex, influenced, it appears, by an 

 opinion that the humours of the eye are combined so as to 

 correct the dispersions which each alone would produce in 

 the different kind* of light, contrived to combine two 

 lenses of different kinds of glass in such a way as to form 

 an im a<je which wns free from colours: it is' added that 

 telescopes with such object-glasses were in the poss 

 of several individuals many years afterwards, Unit. Muff., 

 .r. 17!*l: I'liil. Mag., November. 17SIS. 



In 17J7 Kuler. guided also by the constitution of the 

 -itiility of forming a lens com- 

 pounded ol" two hollow spherical segments of g ';\*~. in- 

 closing water between their concave sides, which should 

 be free from the (-dramatical and spherical aberr;. 

 and in investigating the curvatures he assumed that the 

 logarithms of the terms expressing the ratio of the 

 tion of a mean ray in passing from nir into glass, and from 

 air into water, were proportional to the logarithms of the 

 terms expressing the ratio of the refractions of red ia\ - in 

 the same media. He was not able to obtain from any 

 artist a Ien of this nature, in which the proposed end wns 

 accomplished, and Mr. Dollond [Dou.nM>]. in a short 

 paper which is printed in the ' Philosophical Transactions' 

 17-">i). contested the justness of Killer's principle on the 

 ground that it was contrary to one which he conceived to 

 be founded on the experiment* of Newton. 



Hut M. Klingenstierna. a Swedish mathematician, hav- 

 ing soon afterwards, in a Mc'moire which was sent to the 

 Acad'mie des Sciences, pointed out that the principle 

 which had been adopted by Dollond was not conformable 

 to the acknowledged laws of refraction, the latter deter- 

 mined immediately on havin. to experiment. 

 Kither euided by the object-glasses constructed under the 

 direction of Mr. Hall, or from a sctics of experiments 

 made by himself on the refraction of light in we.] 

 crown and flint elas- 



convex le- Mation with a concave 



I*m of the latter kind, the ray* of the different colours in 

 each pencil of liirht, after refraction through both, might 

 be made to um cus, and 1 



of the object nearly free from colour. For tin 

 discovery Mr. 1> 



f'opleian medal. In I7' ; *> Ins sou, Mr. 1'eter Dollond. di- 

 minished the aberration of light on account of t he spherical 

 forms of the I> ombining together 1 



- of crown ghuw with a concave lens of flint glass 

 between them: this construction is particularly nd\ an- 

 us, by the increased aperture which it allows when 



the focal length of the compound lens is short. 



For several years after the telescopes thus improved by 

 Dollond had been in general use. Eider continued to 



< tlmt all kinds of glass differed but little trot.. 



iicir dispersive power, and he 



cc.s of the Kti; : merely to a for- 



'e determination of the curvature of hi* lenses ; but 



having, in the year 1704, received information that, by 



the addition of lead, -glass hi 



been obtained whose dm- 



' lie eom- 



lot former opinion ; 

 of the 



. tn they were c.nu 

 The most eminent inatheinaliciii' 

 and in this country. ha\e subseo,nc! 

 scientific principle*, UM curvatures w! 



to the surfaces ol lenses, so that, th> , 

 compound lens being assumed, the chronwtii 

 nciil aberrations ma\ 

 The Hiiangcui. -cs for the eye- 



is of no less importance ' 

 object-glass : and Hir. 



in order to dimmish the retraction of liirht at the, surfiicott- 

 to sin - 



d U'U-ftcuj lenses, of cuch curvatures 



lliat 1lu> whole icfraction. or tin 

 dent and emergent niv in tin- former con-' 

 be divided between the two lenses. 



One mode of effecting this purpose is to pla. 

 eye-irlass, or that win > -t to thcohjci'. 



iuliTcept the pencils comimr from the object -<;la.si> 1 

 the rays are muted, and tin _re is formed afti 



refraction of the light in this lens : the second 

 then placed so that the rays falling 

 crossed at the place of the imaire. are made t< 



. rallcl to one another. A microim b e ap- 



jilied to such an eye-piece, -nice any chanire in the place 

 of the lens which is \, lie . \. would doranire its 



adjustment, : tin 



achromatic, and they have the greatest i Id of 



view: they have therefore -inn-ted for the pur- 



pose of merely viewing the celestial bod 'lond, 



Ramwlen. and Frauenhofer. Mr. Kamsden was the first 

 who constructed eye-pieces with two lenses which 

 capable of bcitiLT used with a micrometer: this lie accom- 

 plished by placing the tube containing those lenses so that 

 the rays in the pencils, alter crossim: at the focus 



'-L r las<. fell in a divenrini: state upon the first 



refraction in both, entered the eye m 

 parallel directions. 



With both these kinds of eye-pieces the object appears 

 to be inverted ; but eye-pieces with three lenses, by which 

 the object is made to appear in the erect position, had 

 been proposed byKheita: these being found dele 

 Mr. Dollond endeavoured to improve upon the construc- 

 tion by dividing the refraction nt the first and thii.i 

 glasses between two lenses, according to the method recom- 

 mended by Hnygeus, and thus be formed with 

 five lenses. Hut some light is always lost by reflection 

 when it falls upon slavs ; and. in order to diminish this 

 evil. Dollond subsequently, retaining 1 the Ilnyccnian con- 

 struction in the two lenses nearest to the e\e. used but one 

 lens to perform the office of the second and third in the 

 eye-piece with five classes . in rendering the ra\s of each 

 pencil con. had diminished the diver- 

 gency caused by the crossing at the locus of the objeet- 

 glass: he thus succeeded m producing an eye-pii 

 four lenses which was nearly njilnnntif. or free both from 

 the chromatical and spheiieal aberrations: and such are 

 the telescopes now in common use lor viewing terrestrial 



objects. 



The chief improvements, if they may be so called, which 

 have since been made in dioptric telescopes, consist in the 

 means v. Inch have been adopted to remove' those aberra- 

 tions ) d the natures of the dill- 

 media winch hti\ <l lor this purpose bv I 'r. Hlair. 

 Sir David Hrewster. and Mr. Harlow. [ 

 in the- article TBLKSCOPB. 



Attempts have been mndc by M. f'hevalier to diminish 

 'rrnfions by means 'of two achr* 



'in distance Iron: the tube : 



and by Mr. Rogers of Leith. b\ convex lens of 



plate glass, in combination with a double achromatic lens, 

 the convex lens being of plate-elan*, nnd t 1 

 lens of flint-glass. This last gentleman proposes to unite 

 I and violet r. he object by a 



proper distance betw. le double lens 



and t. Hie spherical aberration either bv giving 



proper cur\ the surfaces of the compound lens, 



or by ; > lenses at a small distance from 



other. (Memoir* of the Atlrtm. N*-., vol. iii., part 2.) Dr. 



