TELESCOPE. 



really is of itfelf, and lliat of jjie bcft conftruftion. We 

 have referved an account of the theory of the compound mi- 

 cro/cope until we arrived at this article, on purpofe to (hew 

 the intimate conneftion that it has with the refrafting tele- 

 fcopt', which, it will be feen prcfently, is alfo the cafe with 

 a compound refleHwg microfcope, that compofes a portion 

 of both the Gregorian and Caflegrainian tclefcopes. Firft, 

 let a b {Plate XXV.j^^f. 8.) be confidcred as an objeft to 

 be magnified for examination by a compound microfcope of 

 the fimpleft conftruftion ; let .5^/ be the fmall objcft-ffhfs, 

 of which i is the folar focas; then as the radiant objedl a b 

 is at a fmall diftance from the lens df, beyond its folnr or 

 principal focus, the incident rays coming from it will con- 

 verge flowly after paffing tiirough this lens, and confequently 

 the conjugate focus at the other fide of the lens will be re- 

 mote, as at A B, where an inverted image, A B, of the 

 objeft a b will be formed ; and if the objeft is brought 

 nearer to the folar focus j, the image A B will recede with 

 a linear amplification, for it always fubtends the fame angle 

 at e, the centre of the objeft-lens, that the objeft fubtends 

 at the fame point ; it is therefore obvious, that the linear 

 amplification of the image, compared with the length of the 

 objeft, will be as their refpeftive diftanccs from the objeft- 



C e 



lens, -aiz. as — ; and, confequently, the farther the miage 



recedes, that is, the nearer the objeft is brought towards 

 the folar focus i, the more it will enlarge, which prin- 

 ciple is the bafis of both the magic lantern, and folar as well 

 as lucernal microfcope. Let us call the ratio of the objeft 

 to its image i : 5, as in our figure ; then if D F be a double 

 convex eye-gb.fs, placed fo that this image, A B, may be 

 in its principal focus, the rays of light coming from it, now 

 confidered as a radiant, will, by paffing through this ler.s, 

 become parallel, in which ftate they will enter the eye at I, 

 and after converging to a new focus on the retina, will there 

 make a pifture of the image of the objeft, but in a reverfed 

 pofition. The principal pencil of rays coming from d and 

 / of the objcft-lens, will meet at C, the centre of the 

 image, and diverge till they come to the eye-lens D F, 

 where they are made parallel, and where they define the 

 fize of the eye-hole in the cap of the eye-piece; while 

 the angle of vifion will be G E H = B E A. In this 

 fituation, the image A B is magnified by the eye-glafs in- 

 verfcly as its focal diftance, that is, as many times as F C 

 is contained in ^ C ; for the vifual angle B E A, fub- 

 tended by B A, exceeds the angle B e A, fubtended by the 

 fame line B A ; and, confequently, its oppofite angle b e a, 

 fubtended by the objeft, is in the ratio of C ^ : C E ; and 

 alfo, when C E = r c, in the ratio of C e : c e ; and the 

 whole amplification wrill confequently be by compounding 



C e' 



the ratios = pc-= . But in this conftruftion the field 



CE X ce 



of view is fmall, though the power is great ; and the colo- 

 rific efFeft of the prifmatic aberrations, as well as the indif- 

 tinftnefs and diflorfion of the figure of the objeft, are fully 

 experienced. To do away thefe impediments to a pleafing 

 view of the objeft, a fecond lens was introduced into the 

 eye-piece, as M N, in ^^. n. Pto XXIV., the original 

 intention of which was, principally, to enlarge the vifible 

 area, or circle of vifion, which it did effeftually, while, at 

 the fame time, it diminithed the power, and in fome meafure 

 the fpherical aberration, though the latter advantage does 

 not feem to have been contemplated ; and in this ftate the 

 compound microfcope remains in the prefent ordinary con- 

 ftruftion, one of the three Icnfes, df, being the cbjeS- 

 Icns ; the fecond being the amplifier M N ; and the third the 

 Vol. XXXV. 



eye-glafs G H. Now if we compare the compound celcf- 

 tial eye-piece in fig. 8, before difcribcd, at having llic fame 

 power with the fingle imaginary lens E E in the fame figure, 

 we fhall fee that the only difference in tlic two arrangements 

 is, that the image in Jig. 11. is between the Icnles, but in 

 Jig. 8. beyond them both. We have di-monftratcd the advan- 

 tages of the co:nl)ination in^"^. 8, and have (liewn that thofe 

 advantages will continue, if the image be formed between the 

 lenfes ; and alfo that making th." diftance between the glaffes 

 to exceed the focal diftance of the eye-glafs, will bring the 

 imaoje into this intermediate fituation, as is aftually the cafe 

 in the bcft achromatic telefcopes, with both the celeftial and 

 terreftrinl eye-pieces ; particularly when the wire, or cob- 

 web micrometer, is not ufed. If then we confider the ob- 

 jeft a b mfg. II. to be the fmall m primary image of a dif- 

 tant objeft, formed in the focus of the achromatic objeft- 

 glafs, the image A B will become the image of an image, 

 or fecondary image, in a contrary' pofition ; and this is the 

 one aftually viewed in the teiTcftrial tube of a teleCcope. 

 Let us in the next place conceive the terreftrial tube to have 

 only the three glalTes that compofe the arrangement of the 

 compound microfcope, and it is obvious that the image a b 

 will be rendered as diftinft, and as much enlarged in it, as 

 the objeft a b, of fimilar dimcnfions, would be in the like 

 compound microfcope. Thus have we a terreftrial eye- 

 piece with an arrangemer.t of three glaffes, which magnifies 

 greatly, and, fo far as the pair of eye-glaftes are concerned, 

 is achromatic ; but with refpeft to the objeft-lens <f/, (which 

 might indeed have been made achromatic by a balance of con- 

 trary difperffje powers, on Dollond's plan of an objeft -glafs 

 for a telefcope), there remained room for improvement ; and 

 this has been efFefted by the fame principle of divifiwn of 

 the aberrations, that contributed to the improvement of the 

 celeftial eye-pieces 'vith two lenfes. 



Plate XXVIII. Jig. 15. (hews a combination of two 

 plano-convex lenfes, that perform the office of one double 

 convex lens in a compound microfcope, or terreftrial eye- 

 tube, in which the lens A is placed next the objeft tn a 

 microfcope, or image in a telefcope, with its plane face out- 

 wards, and the lens B is placed at a diftance from A, that 

 exceeds the focal length of either of the two lenfes, and 

 that is alfo greater than the diftance between the two eye- 

 glafles i but the proportions vary with circumftances. In 

 the patent micrometrical telefcope of Harris, in which the 

 eye-tube is 74 inches long, the locus of the eye-glafs is \i, 

 that of the field-glafs, or amplifier, li, and the diftance 2^ ; 

 while the lenfes of the eye-end are both menifci ; the outer- 

 moft lens having a focus of I,', and the inner lens one of 1.2 

 or i-r,, at a diftance of 2,^. This novel form of a terreftrial 

 eye-tube is found very good for a (hort telefcope, and anfwers 

 equally well for any variable length of focal diftance of the 

 patent objeft-giafles ; and when the telefcope has its focal 

 length invariable, the di.Terence between each fcparate pair 

 of lenfes may b? varied at pleafure, and then the power of 

 the whole combinntion will vary with the variations of this 

 diftance. In all other tclefcopes of the refrafting kind, the 

 two cye-glafFes, as well as the pair of lenfes at the remote 

 end of the terreftrial tube, near the primary image, are all 

 plano-convex, as we have fticwn ; and that combination 

 which fuits a (hort telefcope, will generally fuit a long one ; 

 but frequently that which is made purpofely for a long one, 

 will not fuit a ftiort one. A very good 12-inch terreftrial 

 eye-tube, for a day and night telefcope of two feet length, 

 that we lately examined, has the proportions in the eye- 

 tube fomcwhat different from telefcopes with larger power : 

 the eye-lens has a focus of 2 inches ; the amphfier 3i, at 

 a diftance of 3 inches; and the thiid and fourth lenfes are 

 L 1 rcfpcftivcly 



