TELESCOPE. 



fioned by tlie freedom of flkling, one within another, a 

 clampinjf fcrew at E (jxcs them, and the flow motion pro- 

 iluccd l)y the fcrew F, hniflies the obfervatioii inaltitiule, as 

 tlie liandle '\n fg. 5. does in azimuth; and both flow mo- 

 tions can be maiiajijed, one with each liand, at the faine time. 

 In the prefent reprefcntation, the ctlcllial eye-piece H is 

 fcrewed into its fmall tube, which bears a concealed rack, 

 that is afted on by a pinion on the axis of the thumb-piece 

 G, which maybe made more or lefs tight by a fcrew in the 

 "middle of its plane, and which adjulls the eye-pieces for dif- 

 tinft vifion. There may be any number of various celellial 

 eye-pieces, but two or three are as many as are ufually de- 

 livered with an inftrimient of this fi7.e. The objjd-glafs 

 fcrews into the mouth of the tube at B, and is fo lixed by 

 trial, that the moil dillinft view of an obji rt is had when the 

 fcrew is carried home, in which fituatioii the receiving fockct 

 is fixed by tiie maker ; fo that unfcrewirig the objed-glafs 

 at any time does not injure the inllnmient. The centre of 

 motion in altitude is at a joint above C, and the iteadincfs of 

 the tube A 15 will depend on the dillance of this joint from 

 the fyftem of concentric tubes F, E, P, which may be more 

 conveniently placed towards the eye-end than towards the 

 obje6t-cnd of the main tube, and with equal effeft. When 

 the cylindrical piece beyond the joint of the lowed tube at P, 

 is withdrawn from its hole in a cock, attached to the vertical 

 cylinder, the tubes will pack into one another, and the cylin- 

 drical end-piece will enter the hole of the cock T, under the 

 main tube, and remain out of the way of injury, parallel to 

 this tube. The tube I K, in Jig. 2, fcrews at the end I, 

 into the fame place that the celeftial eye-piece H now occu- 

 pies, and is called the terreflrial tube, or terrellrial eye- 

 piece, becaufe objefts are feen in their diredl pofition through 

 it, which through the celeftial eye-piece are feen inverted. 

 Near the end I, a pair of glaffes, called thc^^iW-glaffes, are 

 fcrewed, and the end K contains the pair of glafles which is 

 denominated the eye-piece. We have already fhewn that 

 thefe two pairs of glafles conftitute an achromatic eye-piece, at 

 the fame time that they erett the inverted image formed by 

 the objeft-glafs in the fmall tube between H and A ; and 

 when this image is confidered as a real objeft, then the ter- 

 reftrial tube is a compound microfcope of the beft conftruc- 

 tion. This mode of defcribing the arrangement of glafles, it 

 is prefumed, will be more eafdy underllood by thole readers 

 who underlland the conftruftion of a compound microfcope, 

 than any other explanation that can be given. The tube L, 

 '".Af- 3> '^ ^" open tube, which is fometimes made, by par- 

 ticular defire, to receive at its end L the eye-piece, now 

 fcrewed into the tube I K at K ; and then, as the empty tube L 

 Aides in the tube I K, the diftance between the pair of field- 

 glaffes and pair of eye-glafles may be varied at pleafure ; and 

 as the magnifying power of the coinpound microfcope varies 

 direftly with this diftance, it is evident that the power of 

 the telcfcope thus conftrufted will vary in like manner. 

 But we have fliewn above, that the power of the telefcope 

 may be varied alfo by varying cither the pair of field- 

 glafles, or thepair of eye-glaffes ; hence, when a great variety 

 of powers is dcfired for the fame inftrument, different pairs 

 of field and of cye-glafTes may be adapted to the fame ter- 

 teftrial tube with very little additional expence ; and mjig. 4. 

 we have given three different pieces of fliort tube, containing 

 male or female fcrews, or both, which are called adapters, 

 by means of which the celeftial eye-pieces may be adapted 

 as eye-pieces to the terreftrial tube, fo as to gain a great 

 increafe of power for particular purpofes. When the 

 adapter M, in_/f^. 4, which has both a male and female fcrew, 

 is fcrewed into the end K, of the terrellrial tube mjig- 2, the 

 celeftial piece W.,Jig. l, may be fubftituted for the pair of 



eye-glaffes belonging to this long tube, whenever occalioii 

 may require ; or the pair of terrellrial eye-glalfes may be 

 made a celeltial pair, on occafions when a low power and 

 enlarged field arc wanted. The adapter O, in f>g. 4, has 

 two dlffimilar female fcrews, the fmaller one of which 

 fcrews upon the long tube at the end I, while the larger end 

 receives the outer end of the celellial ej-e-piece H, in_/f^'. 1, 

 and converts it into a pair of field-glaffes, for which it may 

 be fubftituted, to get the great-'ft jKjflible jiower, with a high 

 magnifier alfo at the end K, or rather at L, with the fliding 

 eye-tube ; and in this way the power may be increafed fo 

 much, that all light will difapjiear, and tlie inftrument, con 

 fequeiitly, will tlien become iifelefs : but it is better to have 

 .idditional ])alrs of proper rKld-glan"es, than to fubftitute 

 eye-glaffes for this purpofe, b.-eaufe the arrangement of the 

 focal diftanccs of the lield-glafl'es is different from that of the 

 eye-glaffes, when they arc arranged in the beft manner, as we 

 have explained under our laft feition. The adapter N hai 

 two male fcrews and a female fcrew, one of which male fcrews 

 will lit the tube at H,/?^. I, and the other the tube at K,/[f. 2, 

 or \.,Jig. 3, and the fcm-ile fcrew will receive Troughton's 

 micrometer in either place, or any eye-piece having a mother- 

 of-pearl micrometer, even though it may belong to another 

 teleleopc. Thus the adapters, which are limple in their con- 

 llrui^'tion, of little expence, and very portable, afford a 

 variety in the ufe of a telefcope, that is at the fame time 

 both ufeful and entertaining ; and we have been the more 

 minute in our defcription of them, becaufe they have never 

 before been brought into public notice. The powers of this 

 telefcope ufually vary from 25 to 100 without the adapters, 

 as they are made by opticians ; and opticians are no advo- 

 cates for adapters that increafe the powers tao much ; but 

 for certain purpofes the power may be .lugmented to about 

 120 with diilinftnefs and tolerable light ; but then it muft 

 be recoUefted, that the field of view will admit of only a 

 fmall object, as well as little light, when the power is aug- 

 mented out of due jiroportioii. 



Fig. 6. is a reprefenlation of a live-feet achromatic refrac- 

 tor, mounted in the moll ufeful and convenient manner for 

 making either celeftial or terreftrial oblervations, and has all 

 the appendages which we have juft defcribed as belonging 

 to the thirty-inch refraftor, when made in the beft manner. 

 A B, as before, denote the main tube, which has a dia- 

 meter of 4-I inches ; and inftead of one fet of fliding con- 

 centric tubes, here are two, inferted into the cocks P and P, 

 of a three-legged Hand of mahogany, of which two legs 

 only are feen in the figure, and thefe Ihortened, fo as to fall 

 within the room allotted them in the plate. The conftruc- 

 tion of this ft and has been defcribed under the article 

 Equatorial Stand, with a reference tojfj. 5. Piatt XIII. 

 of our prefent feries of jjlales ; therefore we ihall fatisfy 

 ourfelves with fuch a fliort defcription of the conflituent 

 parts here, as will limply enable the reader to underftand 

 their ufes. The nulled nuts (J and R, attach the main 

 tube A B to the wooden ftand ; and the tubes A P and A P 

 keep it Heady from vibratory motion : the femi-circlc of 

 brafs between Q and R, moveable about its centre, is racked 

 at the concave part of its circumference, fo .is to fit the 

 fcrew on the axis of the handle U, which we have m.ide 

 fhort, to avoid confufion in the figure ; therefore, when the 

 fcrew is preffed clofe into the notches of the rack-work, a 

 revolution of the handle U, in either direclion, will produce 

 a correfponding motion, in elevation or deprcflion, in the 

 telefcope borr« by this fcmi-circle, while the vernier and 

 divifionsoii the face of the femi-circle indicate the quantity 

 of elevation, when zero is properly adjufted. The manner 

 in which this mcchanifm adis, and alfo the method «f 

 M m 2 producing 



