TELESCOPE. 



and, therefore, an increafe of focal diftance of the large fpe- 

 culum, witliout a proportional increafe of aperture, will 

 anfwer no good purpofe, but will render tlic tube unmanage- 

 able. When the aperture of a CaflTcgrainian or of a Gre- 

 gorian telelcope is to that of a Newtonian as 7.5 to 6, it has 

 been proved that they have equal liglit witii tlie fame powers ; 

 the Newtonian having the advantage, in confequence of tlic 

 obliquity of tiie angle of rcfleftion of the fmall plain fpe- 

 culum. 



Sir Ifaac Newton's conftruAion of the reflefting telefcope 

 differs from both the Scotch and French in this refpeft, that 

 the large concave fpeculuni is entire, and that the fmall one 

 is quite plain, and placed at an angle of 45'', a little (liort 

 of the focus ; fo that the converging rays come to a focus 

 between the fmall fpeculum and the fide of the tube near its 

 fupcrior end, as feen in^_u^. 4. of our lafl plate. Sir Ifaac 

 had diicovcred, in his experiments on refleftcd light, that 

 more rays are reflofted in an oblique tlian in a perpendicular 

 dircdllon ; and thai, confequently, there would be more 

 liglit returned to the eye by a fmall fpeculum fet at 45"^, 

 than would be if the angle of reHeftion were greater. In 

 this inftrumcnt, the theory is much more fimple than in either 

 of the preceding inilruments, on accoimt of there being but 

 one image, h i, formed by the incident rays a b and c d, 

 ;iftcr two refleftions, one of wliich takes place at the large 

 metal B D, and the other when the rays are in a ftate of 

 convergence, at the fmall plain fpeculum E F ; f o that the 

 whole length of the focal diftance of the large metal B D, is 

 B F -)- F f , or D E + E ? ; and this diftance, divided by 

 the focal diftance of the fmall eye-glafs G H, reaching to e, 

 gives the whole power. This calculation is as fimple as in 

 the aftronomical refrafting telefcope above explained, and is 

 analogous thereto. In the inftrument firft completed by fir 

 Ifaac, the eye-glafs was a plano-convex, with the plain face 

 turned to the eye, and ^th of an inch focus, while the focal 

 diftance of the large fpeculum was 6^ inches, and its 



aperture i^; lience its magnifying power was — '—, or 



6i 



= 38. This was at the time confidered a good propor- 



tion between the power and aperture, and a table was con- 

 ftrufted for different focal diftances upon this radical pro- 

 portion ; but it would anfwer no purpofe to copy this table 

 at a time when the reflefting telefcope, in every conftruftion, 

 is brought to nearly a ftate of perfeftion by the fucceffivc 

 improvements of different artifts. In this way of pro- 

 ducing the image, the pofition is inverted ; and the only 

 mode of increafing the power with the fame eye-glafs, is by 

 l^gthening the tube and focus of the large fpeculum ; or 

 with the fame large metal, by ftiortening the focus of the eye- 

 glafs. In this telefcope, any of the eye-pieces, fimple or 

 compound, may . be applied at pleafure : and if the large 

 fpeculum be made of the beft metal, of a proper parabolic 

 figure, and with a good polifli, the image will be ftiarp and 

 well defined ; and as there are no colorific rays in a feparated 

 ilate, the charge, or power of the eye-piece may be great m 

 proportion to the focal length of the large metal, whicli is 

 the diftinguiftiing charafter of this conftrudtion, particu- 

 larly when the fmall fpeculum is perfeftly flat and well po- 

 lifhed. The principal objeftion to this, as a portable inftru- 

 ment, is its unmanageable length, which was firft given it 

 by Hadley, wlio out of a pigmy made it a giant, and afto- 

 nifhcd the philofophic world. The length of tlic tube was 

 maae fix feet, in which was included a metal of fix inches 

 aperture, and 625 focu? ; and Newton's power of 38 was 

 Voi.. XXXV. 



increafed to 230. See Phil. Traiil. vol. xxxii. p. 30J, m 

 Abr. vi. p. 165. 



In fir Ifaac Newton's reply to Caflcgrain'i claim of fiipe- 

 rior adwinta;ns, he ftates, firft, that tlicre will he more light 

 loft by refh'tlion from a fmall convex fpeculum, than liiin 

 a plain fpiculuni of an oval ftiape, and pl.iccd in an obliquj 

 pofition ; fecoiidly, that the convex fpeculum will not rc- 

 fleft the rays fo truly as the plain one, unlefs it be of an 

 hyperbolic figur'^ wliich is diiiicult to form, and even then 

 will refleft only ihofe rays truly which rcfpcft the axib; 

 thirdly, tiiat tiic errors of the convex furface will be aug- 

 mented by tiie diftance tlirough which they pafs before they 

 reacli the eye ; fourthly, that the errors of the convex furface 

 will be increafed by tlie difleftion or bending of the figure 

 from the points where the incident rays ought to fall ; 

 fifthly, tliat on this account the figure is rcquirtJ to be more 

 perfect than art can make it ; fixlhly, tliat the errors of the 

 laige metal, which is confidered to be fpherical, will be fo 

 augmented by rcfleAion from the fmall convex metal, that in- 

 diitinclnefs will enfue, fucli as will not allow either a great 

 aperture, or a high charge ; and laftly, that as the fm j11 B-.etal 

 contributes to increafe tnc power, an over-charge of power, 

 compared with the aperture and focal length of the large 

 fpeculum, will be unavoidable, fo as to produce very ob- 

 fcure and confufed vifion ; for if the fmall metal be made 

 with a laiger radius, in order to diminiih the power, tpo 

 many of the incident rays will be intercepted ; and if the 

 charge of the eye-glafs is diminiftied, the area of the field cf 

 view will be fo far diminiflicd, as to render a fm.ill objefl 

 only vifible, and that diflicult to find. Thefe might be ob- 

 jeftions a century and a half ago ; but moft of them have 

 yielded to fubfequent improvements in the nice ait of carting, 

 grinding, and polifliing of fpecula, which we explained under 

 the word Speculum, and in the formation and arrangement 

 of the Huygenian eye-piece, which we have faid is pecu- 

 liarly fuited to refiefting tekfcopes ; though fingle Icnfrs 

 will do very well wlien the fpeiftator confines the axis of his 

 eye to coincide with the axis of the lens, fo as not to pro- 

 duce diftorfion in the figure of the image viewed. It may 

 be proper to mention further here, that the fmall telefcope 

 called the finder, attached to telefcbpes of confiderable 

 power, was firft propofed by fir Ifaac Newton, to remedy 

 the dilliculty of finding the objeft with his reflcfting tele- 

 fcope ; and Defcartes hath defcribed it in his " Dioptrics" as 

 anfwering the fame purpofe when applied to his beft telefcopes. 

 Indeed objedtions well founded and rationally ftated have led 

 to various improvements in the mechanical arts, and are never 

 to be difregarded, unlefs they are obviotidy futile. Had fir 

 Ifaac Newton lived to have a peep at the inftrument which 

 next claims our attention, he would no doubt have been 

 highly gratified at the progrefs which the art of conftruftinc 

 telefcopes has made fince his fix-Lnch reflcftor, with its ball 

 and focket, was mounted over a candle-ftick, or a fmall 

 pillar greatly refembling this domeftic utenfil ! And yet, to 

 do juftice to his inventive genius, if we may apply figurative 

 language to fuch a fubjeft, the feeds of all the fruit that has 

 fince been matured were contained in Iiis primitive little ker- 

 nel ; and we are proud to claim the Newtonian as the Eng- 

 Vijh produftion. 



" Though laft not leaft," the Hcrfchclian telefcope now 

 offers itfelf to our confideration, which we might with fome 

 propriety call the Gc«-man telefcope, inafmuch as the cele- 

 brated contriver of its llupendous mcchanifm is a native 

 of Hanover : it was hov.-ever conftruftcd in England, and 

 by Englifti workmen, except fo far as the ingenious 

 knight of the Royal Hanoverian Guclphic crdcr lent his 

 M ni powerful 



