TELESCOPE. 



altitude by a fmall quadrant iixcd over it, aiid the corre- 

 ^oiideut index is regulated anfwerable to the ft retching of 

 the cord of communication, by obferving the Hrft known 

 liar that paflTes of Flamftead's catalogue. By this means, 

 what has been done and what is to do is diftinftly fecn 

 by the fheets. In this way, many hundred nebulas have been 

 difcovered, not only unknown before, but which no ordinaiy 

 telefcopes will reach. The fpeculum of the great telefcope 

 of 40 feet is cad, but wasnot got home ; it is four feet diame- 

 ter, and about lojo lbs. weight. Mr. Herfchel tells me 

 there is a wanlioul'e in Thames-llreet, where they keej) for 

 fide metal ready made into ingots, of which they have two 

 forts, what they call white metal and bell-metal ; I fuppofe 

 fuch as the bells of clocks are made of, but he did not know 

 exaAIy their compofition : for his fpeculum, they put two 

 ingots of bell-metal to one of white metal. He thinks it a 

 lower metal than what he ufed for his former fpecula of 19 

 inches, viz. yf ounces of tin, to 20ouiices of copper. I am not 

 fure, however, whether I remember right, but you probably 

 ■^vill guefs. He does not propofe it to magnify more than 

 the prefent one of 19 inches, but to. take the whole advan- 

 tage in light, he makes all his fpecula flat upon the back 

 fide. The thicknefs of this lail great one at the edge was to 

 have been two inches, but by fome (lirinking in the mould, 

 and particularly in the middle, I underlland it is not there 

 above I J inch, and alfo Icfs at the edge than it was to have 

 been, fo that it is hollow in the back as well as face ; but as 

 it came pretty well upon the face, he promifes to make 

 ufe of it ; and when he has got thus furnifhed, he promifes 

 to caft another, having duplicates of all he makes, fo that 

 while one is in ufe, another can go to the polifher. They are 

 made to be enclofed in brafs boxes, and their weight lays 

 limply upon feveral thickneffes of cloth, and are polifhed in 

 tliefe boxes, and are made to go in and take out fo con- 

 veniently, that they are very frequently put into their tubes 

 and tried with an objeft while under the operation of poli fil- 

 ing ; and to thefe frequent trials he afcribes the principal 

 caufe of his fuccefs in thefe operations. 



****** 



I remain, dear fir, 

 ever your's, 



J. Smeaton. 



We have only to add farther, on this fubjedl, what we 

 leai-nt in a converfation with fir William Herfchel, that he 

 prefers fingle lenfes, before what are called achromatic eye- 

 pieces, from an idea that more light is thus had for both 

 his 20 and 40 feet refleftors, and that greater poiuer may 

 thus be obtained for his fmaller inftruments. We have, how- 

 ever, to regrot, that his mode of giving the parabolic curve 

 to the great fpeculum, by mechanical means, muft for the 

 prefent remain a fecret, for the difclofurc of which we feel 

 th.it we have no right to afli, while thire is an exifting 

 manufactory that might be injured thereby. The peculiar 

 advantage of the Herfchelian conftruftion is, that there is 

 no light loft by a fecqnd reJlcHion, and that the large quan- 

 tity of polifiied furface reflefts more rays than can be col- 

 lefted by any other means. The weight of the metal, 

 which is very brittle when of the beft mixture, made it 

 neceffary to have a prevailing portion of copper in the large 

 fpeculum, which is, therefore, liable to be the fooner 

 tarniflied, and to require more frequent poliftiing than 

 would have been requiiite, if the beft proportion for bright- 

 nefs could have been preferved in the ingredients of the 

 compound metal : but what is defeftive in quality, is com- 

 penfated by the quantity of poliihed furface. It is hardly 

 neceffary to inform the pradical aftronomer, that when the 



4 



greatcil powers are ufed, both tlie light and the field of 

 view, and confequent time of apparent paffage through the 

 field, are proportionably diminished. We underftood the 

 ingenious and dextrous obferver to fay, that inftruftion 

 and praAice are neceffary to enable any other perfon to 

 follow a ftar or planet with the forty-feet refleftor ; for that 

 a heavenly body feen with one of the higheft powers does 

 not continue in the field more than a few fcconds of 

 time, unlefs the motion of the tube is regulated fo as to 

 keep pace with the app.vent motion of the body ; and this 

 is probably the reafon why few perfons have been in a 

 iituation to form an eftimate of the merits of this tranfcendant 

 inftrument. For the detail of all the parts, fee vol. Ixxxv. 

 of the Philof. Tranf. of London, part ii. 1795. 



5. Oti the Potiiers, isfc After having described the moft 



convenient conftrudlions of a telefcope, of both the dioptric 

 and cata-dioptric kinds, we propofed to fhew how their 

 powers may be praftically varied and eftimated. We have 

 already feen, in our feftion on the theory of telefcopes, how 

 the powers may be calculated, when the focal diiiances of 

 the glafles are known. In telefcopes with one objeft-glafs, 

 or concave fpeculum, and one eye-glafs, the folar foci of 

 which may be called F andyrefpeftively, the power P may 



F 

 be always expreffed by — ; but F varies inverfely with the 



diflance of an objeft viewed, while /remains the fame, there- 

 fore the power P will vary alfo inverfely witli the diftance. 

 So long ago as in the year 1740, Benjamin Martin, to whofe 

 ingenuity the praftical opticians of the prefent day are 

 much indebted, propofed to determine diftances at one fta- 

 tion by this variation ofpoxuer in a long telelcope ; but as the 

 diftance increafed, the proportional elongated portion of 

 the Hiding tube containing the eyc-glafl'es became fo fmall, 

 that the icale was too hmited to be of any real ufe. We 

 mention this circumftance, merely to ftiew that the fame tele- 

 fcope with the fame glafles has its powers natur.illy varying 

 with the diftance, but in an inverfe ratio, until the incident 

 rays become parallel in confequence of the great diftance of 

 the radiant objed; ; hence we may account for the reafon 

 why the famous Short attributed to his telefcopes powers 

 which they did not poflefs, when directed to very diftant 

 objefts. But, generally ipeaking, when we fay that a tele- 

 fcope magnifies fifty times, we are underftood to mean, that 

 it enlai-ges the diameter of the fun, or of fome dijlant objeft, 

 fo many times ; becaufe in this cafe F and / remain both 

 unaltered. (See Lens, 5.) But when the objeft viewed 

 is at no great diftance, calhng the elongated portion of the 

 folar focus e, and the diftance d, Martin has ftiewa that 



F -f- f X F 

 ^ : F :: F -{- e : d, or that = d; and he pro- 



e 



pofed to determine the quantity of e in all fituations by me- 

 chanical meafurement. Now fuppofing the power to be 

 confidered as always detemiined from the folar focus of an 

 objeft-glafs or fpeculum, in telefcopes of the fimpleft con- 

 ftruftion, this power, wliere the objeft-glafs or fpeculum re- 

 mains the fame, can be increafed only by fliortening the focus 

 of the eye-glafs or eye-piece, when it is compofed of two 

 glafles ; but there is a limit in the power of dioptric tele- 

 fcopes conftrufted with fmgle objeft-glafles, which depends 

 on the prifmatic and fpherical aberrations, beyond which 

 limit indijlinllnefs takes place ; and even in good achromatic 

 and reflefting telefcopes, the eye-piece may be fhortened 

 until a deficiency of light renders the increafed power of 

 little ufe, and thus fixes a limit to ufeful power. 



After a power is fixed on, in the ufe of a fimple tele- 



fcopej 



