TELESCOPE. 



powerful affillance, partly in the execution, but chiefly in 

 the contrivance of the mechanical appendage?. The work 

 was immenfe, but royal means furniflied the power that 

 overcame every obftacle. In magnifude, as in power, the 

 forty.feet reflcAor at Slough exceeds every inilrument that 

 human indullry has yet put togetiier, and ftands as a proof 

 of great mechanical (kill diredcd by an enterprifnig mind. 

 Wc trull that we cannot be underllood to derogate from the 

 merit of Dr. (now llr William) Hcrfchel, when we Hate tiuit 

 the idea of giving a fmall degree of obliquity to the large 

 fpeculum of a rcflefting telclcope, fo as to bring the image 

 formed in the focal point out of the body of the tube, 

 at its aperture, originated with Le Maire (fee Ma- 

 chines Approuvees, par I'Acad. vi. p. 6i.) about the year 

 1728 ; for as we know not that this fuggeftion ever met the 

 eye of this illuftrious aftronomer and mechanic, previoufly to 

 his undertaking the Herculean labour of conftrufting a tele- 

 fcope with a fpeculum of forty feet focal diftance, and four 

 feet diameter, it would be unfair not to allow him the credit 

 of the invention as well as of the conttruftion. We men- 

 tion the name of Maire in compliance with our fyftem of 

 tracing, or attempting to trace, from hiflorical evidence, the 

 origin of each mechanical invention that has contributed, 

 immediately or remotely, to the promotion of the mechanic 

 arts. The theory of this conftruftion is eafily explicable, 

 by a reference iofy- 5. of our plate of the Theory of Refiell- 

 ing Tehjcopes, in which, as before, A B C D is the tube, 

 and B D the large fpeculum of the immenfe weight of 

 21 18 lbs.; the incident ra^'s a h and c d, which would have 

 come back to a focus at the point c, in the centre of the 

 aperture, if the axis of the fpeculum had coincided with 

 the axis of the tube ; in confequence of a fmall inclination 

 of the fpeculum, given by fcrews behind, come to a focus 

 near A, at the edge of the tube, where the image of 

 the object is formed by only one reflection, which is the 

 leading feature of the conllruftion. This fimplicity of prin- 

 ciple is very convenient wlien a large aperture is wanted, 

 becaufe the head of the obferver may be placed entirely 

 at one edge of the tube, fo as not to intercept any of the 

 rays at the time of making an obfervation ; but as the eye 

 iooks down the tube in erery ftate of elevation, not only 

 muft the back be turned to the object viewed, but the ob- 

 ferver muft be mounted nearly as high as the fuperior end of 

 the tube, in order to make his obfervations : hence various 

 pullies, ladders, fcaffolds, &c. became neceffary to enable the 

 obferver to adjuft both the inflrument, and at the fame time 

 his own pofition, all which will be bed underllood from the 

 drawing, when we come to explain the particulars of the 

 ronJirnSion hereafter. The power of the Herfchelian, as 

 that of the Newtonian telcfcope, is obtained from the ratio 

 between the focus of the fpeculum and tlie focus of the eye- 

 glafs, which in this inftrument is not very (hort, though 

 the image is formed by limple refleftion. The mode of va- 

 rymg the power is the fame, therefore, as in the Newtonian 

 refledlor, and requires no further explanation. 



4. The Conjlruflicn of Tele/copes As we have now given both 



the hiflory and iht-ory of telefcopcs at confiderable length, we 

 fhall not be under the nccefllty of dwelling long on each of 

 the feveral conllruftions ; particularly as a reference to the 

 drawings which wc have given, and which are moftly original, 

 will exhibit to the eye more precife information on this part 

 •f our article, than any the minuteft detail would do, unac- 

 companied by fuch vifible reprefentations. Now that the 

 long aerial telefcopcs are no longer in ufe, we fhall not fill 

 our pages by defcribing the different kinds of mechanifm 

 that were applied for rendering them ufeful in obfervations, 



6 



by Huygens, Perrault, Scbaftian, Mairan, and others ; mofl 

 of which are defcribed in vols. i. v. and vi. of the " Ma- 

 chines Approuvees par I'Academie, &c." to which we 

 beg leave to refer the curious reader who wi(hes to know the 

 particulars. All the fupports for long telefcopcs had necef- 

 farily one property, which is dehrable alfo in ftands that are 

 made for modern telefcopcs, hut which is frequently ne- 

 glected ; and that is, that the objeft-end of the telcfcope was 

 Iteadily fupported by fome point of reft near the remote ex- 

 tremity, where the rays were incident. Indeed various 

 Hands or mountings, as they are fometimes called, have been 

 contrived for the convenient fupport of a telcfcope, when it 

 is too heavy for the pocket, and incapable of Aiding by con- 

 centric tubes into a portable form ; but in every ufeful 

 ftand the following properties ought to combine : firft, the 

 inilrument held by it (hould be kept firmly in its place, if of 

 the refracting fort, fo that the image may have no vibratory 

 motion unfavourable to diftindt vilion, occafioncd by an 

 unfteady polition of the objedt-glafs ; but if it be of the re- 

 flecting kind, then it fhould be fo fufpcnded, that tremulous 

 motions arifing from compaftnefs of the materials, ftich as 

 eafdy tranfmit vibratory impulfes, may be avoided : fecond- 

 ly, a motion in azimuth, and another in altitude, are indif- 

 penfable ; and if the inftrument be bulky, or have great 

 power, in each of thefe refpefts there (hould be both a 

 quick and a (low motion, the former to fave time, and the 

 latter for the fake of accuracy : thirdly, when the inftru- 

 ment is pointed to the required objeft, it (hould not be 

 liable to be eafily moved by any accidental touch of the ob- 

 ferver's hand or body ; unlefs it is managed in a ftate of fuf- 

 penfion, as is the cafe with the larger reflectors : fovirthlv, 

 the parts of the ftand (liould be ftrong enough to bear the 

 fuperincumbent weight, and not liable to get out of repair ; 

 and fifthly, its pofition (liouId be in a lituation not eafily 

 /haken, or moved by the obferver's weight, or that of a by- 

 ilander. When thefe properties are attended to, the exaft 

 fhape and external appearance become matters of fecondary 

 confideration, and each artift may purfue his own fchemes in 

 the conftru£tion ; but in this, as in feveral other depart- 

 ments of the mechanical arts, that work is beft and quickeft. 

 performed, which is done from approved patterns. 



liefraSing We will firft defcribe the refrafting tele- 

 fcopcs reprefented in Plaie XXIX., and (hall then proceed 

 to the reflefting inftruments contained in Plates XXX. and 

 XXXI. ol AJlronomical Injlruments, omitting thofe portable 

 inftruments that are in the hands of every reader^ fuch as 

 opera-glalfes, &c. and that belong more properly to the 

 head of Optics. 



One of the beft ftands for a thirty-inch refractor, by which 

 we always mean an achromatic refraiftor, is that exliibiced in 

 Plate XXl^. Jig. I. ; in which. A B is a tube of brafs, 

 mounted on the tripod ftand of the fame metal C D, and 

 fixed by means of the fcrews Q and R. In the common 

 conftruflion, the horii;ontal motion is at C, at the top of the 

 ftem or cylinder, and the fyftem of tubes Y, E, P, is not ap- 

 plied, fo that there is neither the flow adjuftable motions, 

 nor is the inftrument fteady in any given pofition ; but here 

 the horizontal motion is at D, at the lower extremity of the 

 cylinder, where there is a long bearing for the pivot, with a 

 tightening fcrew underneath the jundtion of the feet, and a 

 clamp S to fix the inftrument in any given direftion. Alfo 

 the handle mjig. 5, with a Hooke's joint, taking the fquared 

 axis of the fcrew at D, gives the flow motion in azimuth, 

 while the Aiding and adjuftable tubes F, E, P, keep the angle 

 of elevation unaltered. Thefe tubes turn on a joint at P ; and 

 when a due elevation is given by the quick motion, occa- 

 fioncd 



