TELESCOPK 



mahogany light frame, four feet fix inches liigh ; the crofs- 

 piece, A B, is fifteen inches long, and the piece C D feven- 

 teen, at the diftance of eighteen from the other ; another 

 frame, E F G H, with parallel fides, nine inches apart, and 

 more flender than the other frame, pafTes through the crofs- 

 bars of the former, in fucli way, as to have an eafy motion ; 

 a cylinder or rod of brafs is fcrewed by its head-piece to the 

 crofs-bar E F, and defcends from M to N, through a wooden 

 fcrew L O, which is hollow within, and cut into a fcrew 

 round its circumference : this wooden fcrew terminates 

 above with a brafs focket and ihumb-fcrcw, which ads as a 

 fcrew of prcffure againft the interior brafs rod : the thick 

 wooden piece L has a female fcrew, a6\irig with the male 

 fcrew of the hollow wooden cylinder L O, but is fo made 

 fall to the crofs-bar A B, by a circular plate of brafs 

 above, that though it will turn round, it will neither .ifcend 

 nor defcend ; confequently will produce an afcending or de- 

 fcending motion in the wooden cylindrical piece L O, aud 

 alfo in the brafs rod M N, held by the fcrew K, attadied to 

 It. The concave piece of brafs I has two ' motions in its 

 ilem, one iiorizontal, and the other vertical, like thofe in 

 the ftem of a fmall telefcope, and receives the eye-end of a 

 long telefcope, to which it is fcrewed, while the remote end 

 is fupported by the branch of a tree, the block of a pulley, 

 an opening in the roof of a houfe, or other elevated part of 

 a building. The adjiiihnents are thus managed ; when the 

 elevated end of the telefcope is made to reft on its bearer, 

 the eye -end adapts itfelf to the inclination by the joint in its 

 rtem under I ; then the whole frame is turned to face the ob- 

 jeft, when the circular motion of the fame fmall ftem yields, 

 and allows the long tube to remain quiet ; and if the tube is 

 not exactly pointed in azimuth to the objeft, the brafs piece 

 P, into which the ftem I is made faft, Aides along a groove 

 made in the front face of tlie crofs-bar E F, until the adjull- 

 ment for azimuth is; complete. This Aiding motion, being 

 manual, may be either quick or flow, as the obferver de- 

 fires ; therefore, when a body in motion is once in the field of 

 view, it may be followed without difficulty, by puibing the 

 fliding-piece P in a proper direSion. The quick and flow 

 motions for adjuftment in altitude are feparate, and are thus 

 produced ; firft the thumb-fcrcrv K is turned back, fo as to let 

 the rod M N afcend freely, till the altitude is nearly right, when 

 it is fixed, and then the piece L is turned, backward or for- 

 ward, as the cafe may require, with the right hand, while the 

 left flowly Aides the piece P, until the objeft is in the middle of 

 the field ; and when diftinft vifion has been properly obtamed 

 by the fmall tube at the eye-piece of the telefcope, the pieces 

 L, and P, held refpeftively in each hand, will always afford 

 the means of keeping the objeft in the proper part of the 

 field ; and though the fupport has but tw6 legs, yet its con- 

 neftion with the fupport at the objeft-end, through the 

 heavy tube of the telefcope, will always keep it in its place 

 when the adjuftments are fettled. It will not be ncceftary to 

 defcribe any more ftands, of which a great variety might be 

 produced, that have been devifed for refrafling telefcopes, 

 becaufe we prefume that our readers will be able, after 

 what we have faid on this fubjeft, to feleft fuch as may beft 

 fuit their refpeftive purpofes. 



We come now to defcribe the portable patent achromatic 

 telefcope, without a ftand, that v/as invented by Dr. Brcw- 

 fter, and is fold under the patent by Mr. Harris, optician, 

 m Holborn, London. The conftru6lion of this telefcope, of 

 which we have already explained the theory, is two-fold, 

 and maybe explained by _/?fj. 4. and 5. Ptez-XXX. In 

 Jig. 4. the tubes are fuppofed to be tranfparent,or otherwife 

 fo divided, that the interior parts may be feen in tlicir rc- 

 ipeftive fituations ; A B C D is the outermoft tube, of ma- 



hogany, with brafs ends, containing an »cIiromEUc objefi- 

 glafs, A D, at its exterior end ; E F G H is the next tube 

 of brafs, fliding fmoothly into the mahogany one, without 

 lateral Oiakc, and may be called the fccoiid lube : into this 

 tube the third tube I K fcrews ; and in its turn receives the 

 fourth, or tcrrellrial eye-lube L O, containing the pair of 

 field-glafTes L M, and pair of cye-glafles NO; all whicli 

 tubes Aide into the fpace of twelve inches, (including the 

 cap,) to fit the pocket. The principal objeft-glafs, A B, is 

 an inch and feven-cighths in diameter, and has a lolar focus 

 of 1 8. J inches ; and if there were no other glaffes but the 

 four contained in the terreftrial, or fourth tube, tills w^iild 

 be nothing more than an ordinary portable or pocket «ciiro- 

 matic telefcope : but at I, in the fecojid tube, is fcrcwrd a 

 fecond obiedt-glafs of the acliromalic fort, the diameter of 

 which is One nich and three-eigiillis, and its folar focus 

 14 inches. When the fecond tube, E F G H, is puAied into 

 the wooden tube A B C D, the fecond objecl-glafs, I, ap- 

 proaches the principal »bjet\-glafs A B ; and when the tiiird 

 tube, 1 K., is alfo puAied in, the two objeil-glafles con-.e 

 nearly in contail at the end A B of the telefcope : in this 

 fituation, the compound focus of the two objett-glalles, by 



F X / 

 the theorem = j, is about tight inches, wliich 13 the 



fhorteft poffible ; and in this ftate of the tubes, when the eye- 

 glaAts are adjulled for diftlnft vifion, the power is the 

 fmalleft pofTible ; but when all the tubes are drawn out, the 

 diftance. between the objcft-glaffes, and confequently the 

 power, will be the greateil poilible, becaufe the converging 

 rays coming from the principal objeft-glafs A B, will nave 

 paffed througli one half of the lubes before they fall on t!ic 

 fecond obje6t-glafs, and undergo a fecond refra<Sion, fo as 

 to come to a Aiortencd focus. In every intermediate pofi- 

 tion of the objeft-glafs I, the power of the telefcope, that 

 is, the compound focus of the two objeft-glalFes, will be de- 

 ternujied according to their intermediate diftance, by the 



F X /" 



theorem = -z^ — -,. Thus in every new pofition of the 



F -h/— a 



fecond objeft-glafs I, the telefcope will have a new power, 

 and thefe powers might be marked by a fcale running length- 

 wife along the fecond and third tvibes which feparate the two 



obieft-elalTes ; but the inventor has made another ufe of this 

 ... ... 1 , 



property, by convertmg it mto an opitcal micrometer : tie tias 



fixed two parallel wires in the focus of the eye-piece, and alfo 

 two points of metal, to include a larger angle, in a direftiou at 

 rin-ht angles to the former ; fo that when one pair includes a 

 horizontal angle, the other will include a vertical one ; r.n 

 experiment is then made by aftual meafurement, of a divided 

 fcale, placed at a mcafured diftance, to afccrtain what is the 

 angle mcafured by the points in the eye-piece, when the ob- 

 jeft is feen between the parallel wires in each of the extreme 

 pofitlons of the fecond objeft-glafs I, and ftrokes are 

 marked accordingly, as the boundaries of the intended fcale, 

 the end of the next contiguous tube being the index ; thus in 

 the inftrument before us, the extreme points or ftrokes of the 

 fcale a b, in fig. 5, meafu.e no' and 218' refpeftively, at 

 an interval of iji inches; and as it has been determined, 

 both from theory and praftlce, (fee Dr. Brewfter's Trca- 

 tife on New Philofophical Inftruments,) that the fcale of 

 meafures is a fcale of equal parts, thefe 15^ inches are di- 

 vided into 108 (218 - no) minute fpaces, while eaclt 

 minute fpace is blfefted into fpaces of 30", which might 

 again be bifefted by the eye, if the adjuftment for vilion 

 could be made fo nicely as to admit of fuch cftiination. 

 Hence it is eafy to conceive how this telefcope will mealure 

 any angle fubtcndcd by a diftant objcA of unknown dimcn- 



fions, 



