TELESCOPE. 



fcope, fuch as admits of lufficient light, and allows a field of 

 fiew large enough to contain the images of tiie ob]c6t to be 

 examined, the magnitude of tliis power may be alcertained 



F 



by different means, befides — , wliicii cxpreffion is better cal- 

 culated to explain the theory than to define the praAical re- 

 fult ; for it is not an eafy matter to meafure precifely the 

 exaff compound {ohx focus of an eye-piece compofed of two 

 glatfes, nor yet that of a finglc lens, when its focus is (liort, 

 and confcquently its fubftaiice confiderablc in thickneis. 

 Neither is it eafy to obtain the exaff power of a tcrrcllrial 

 eye-tube conihufted on the principles of a compound micro- 

 fcope. The firfi. pradical method of meafuring the total 

 power of. a telefcope, that we (hall defcribc, is extremely 

 fimple, and is applicable to telcfcopes of all conftruclions, 

 however complex tlie calculation by theory may be, and 

 gives the refult with very little trouble. Whatever be the 

 diameter of the objcft-glafs or fpeculuni of a telefcope, in 

 inches and parts, tlie diameter of its image, or luminous difc, 

 formed in the anterior focus of the eye-piece, by the con- 

 denfed rays, will bear the fame proportion to that diameter, 

 as tl«a focal length of the eye-glafs or glafles jointly, beais 

 to the focal length of the objeft-glafs or fpeculum ; thefc 

 diameters, tiierefore, maybe fubftituted for the two foci of the 

 refpeftive glafles, or fpeculum and its eye-glafs, in deter- 

 mining the power. Different methods of meafuring the 

 luminous difc have been propoied ; a nicely divided (lip of 

 mother-of-pearl, fixed in a fmall piece of tube bearing a mag- 

 nifier at the oppofite end, forms a fimple inftrument, which 

 has been called the pearl dynamcter, (from liniot.fj.iuir, of power, 

 and fiETpov, a meafure,) and which anfwers the purpofe very 

 conveniently, when (liding within another (liort tube for the 

 fake of adjullment, as is feen mjig. 9. Plate XXIX. 



Suppofe that the difc of a telefcope, with an obje(ft-glafs 

 of 3.25 inches diameter, meafures ^y^ = 1-03°^ ^n inch by 



the pearl dynameter, then — — r= 54^ is the power required 



to be meafured : and if the fame difc had been meafured 

 with a reflefting telefcope of 7.5 inches diameter of the 

 large fpeculum, whatever its conftruftion in other refpefts, 



the power would have been — ^ =: 125. The correftnefs of 



this fimple method will depend on the accuracy with which 

 the refpeftive diameters of the difc and objeft-glafs, or fpe- 

 culum, ai'e taken, and the diftance to which the telefcope is 

 adjufted for diftintl vifion. The powers of the four achro- 

 matic telefcopes, for which we have adapted our Tables I. 

 and II. in the next fe£tion, were taken in this way, when 

 Troughton's micrometer was applied as a celeltial eye- 

 piece, and were determined to be as follow : viz. 



J"- ^ f 2.05- 



30.15 focus \;^^-^ = 



1.077 

 t .05 1 2 



[3-0OO 



t .052 



Thefe powers, if the data had been taken with perfeft ac- 

 curacy, would have been refpeftively to each other as the 

 focal lengths of the objett-glalTes direftly, which they are 

 nearly, or inverfely as the vahies of the micrometrical fcrew, 

 which values have been tabulated, as vrill be feen in our 



45.75 ditto 



63.5 ditto 



118.8 ditto 



30.5 = power. 

 45.3 = ditto. 

 = 63.5 = ditto. 

 120.0 = ditto. 



fubfequcnt fctlion ; thcrefot-c, when the power of one of the 

 telefcopes is obtained accurately by tlie pearl dynameter, 

 the powers of all the others may be had from the micro- 

 metrical values, by reciprocal proportion. Before, liovv- 

 ever, the dynameter is ufed, it will be nccedary to adjul) the 

 eye-piece to didind villon when viewing a remolr objedt, 

 otherwiie the diic will be too fmall, and the power larger 

 than when celcllial obfervalions are taken. Alfo, to avoid 

 millaking the anterior glafs of the eye-piece fur the difc or 

 diiiiiniflied image of the objetl-glafs, a llip of paper may be 

 (luck on the centre of the exterior face of the objecl-glafs, 

 the image of which will appear on the centre of the difc, and 

 alfill the adjullment of the dynnmeler to its true place of 

 diftindt vifion, which is efTential at the moment of taking the 

 exaft meafure of the difc. If one of the celellial eye-pieces 

 lias got a divided flip of pearl, as recommended by Cavallu, 

 to be ufed as a micrometer, the interior I-ns may be taken 

 out, and then the eye-piece will become a dynamcter for 

 meafuring the powers of all the other eye-pieces, whether 

 celellial or tcrrellrial, in the way we have here defcribcd ; 

 but it will be more convenient to u(e one with a Hiding 

 tube of adjullment for diftance, as made by T. Jones, of 

 Charing-Crofs. 



As this dynameter has lately been conftniftcd in an im- 

 proved manner, by the maker we have juft named, and as it 

 has never been dcfcribed, we will here give our readers a 

 (hort account of its improved conllruftion. Fig. 9. of 

 Plate XXIX. reprefents this neat httle inftrument of nearly 

 its full fize, where a, b, and c, are fo many fmall tubes 

 within one another : the (hortcft tube, a, contains the two 

 plano-convex lenfes^and g, which conftitute what we have 

 called the pofitive, or Ramfden'a eye-piece, with the two 

 curved faces oppofed to each other ; and as this eye-piece 

 fcrews into the tube b, near the end g, it may be confidered 

 as a part of this tube, when fcrewed into its place : the tube 

 b has a (lip of the mother-of-pearl, d d, very delicately- 

 made, and fcrewed faft acrofs a diaphragm near its remote 

 end, at fuch a diftance from the lens g, that the fcrew of the 

 eye-piece a will adjuft the pearl for diftinft vifion, as an 

 objcil in the compound focus of the eye-piece, for any eye 

 that may have occafion to ufe it. The !lip of pearl is di- 

 vided into fuch minute parts, that 500 of them are equal to 

 an inch, and yet the eye-piece has power enough to give a 

 clear view of them, and to enable the eye to count the di- 

 viding ftrokes, of which every fifth is of double, and every 

 tenthof four times the length of the fubdividlng ftrokes. 

 When the fcale is rendered clearly vifible and legible, by the 

 fcrew of adjullment, the tube b isinferted into the oulermoft 

 tube c, which has a diaphragm and covered hole at c, and 

 when this hole is uncovered, tube c is brought into contadi 

 with the eye-piece of the telefcope, centre to centre, fo as to 

 receive the pencil of condenfed rays, that iifually enters the 

 eye of a fpedlator ; then, if the image of the objeA -glafs of 

 the telefcope formed at the place of the eye, is not well 

 defined on the llip of pearl, tube b muft be puflied into 

 tube c, till this will be the cafe, and then the number of divi- 

 fions and fub-divifions of the pearl fcale, that the little lumi- 

 nous circle exattly covers, will give the meafure required ; 

 and if the number read be doubled, becaufe they are 500th 

 parts of an inch, they will then be fo many parts out of 

 1000, and will therefore be decimal parts of an inch ; the 

 denominator being confidered ^ 1 00c. 



Another method of afcertaining the powers of a tele- 

 fcope, when a dynameter is not at hand, is by what is called 

 falfe vifion, which requires a little prailice before it can be 

 applied with fuccefs. By this method, one eye views the 

 magnified image of a diftant objed in the telefcope ; and 



the 



