SPECULUM. 



and to have become fiat at the vertex, fo as to be incapable 

 of becoming a proper bed for the bituminous coat that 

 was intended to receive the emery. 



In the telefcope vvliich fir Ifaac Newton firft made, the 

 focal length of the concave fpeculum was 6 ' inches, that 

 of the eye-piece, which was a plano-convex lens, being juft 

 ^th ; but as the metal was neither perfeft, nor very highly 

 polilhed, he thought that the power ought not to be fo 

 highly charged for opaque objefts, particularly as the aper- 

 ture was only i id of an inch. Suppofing the performance 

 of any inftrument to be knowo and approved, the author 

 propofes the fubjoined table to (hew what ought to be the 

 apertures, lengths, and powers or charges of other inftru- 

 ments to perform equally well. In the firft column the 

 length of the telefcope is expreffed in feet ; wliich quantity 

 doubled gives the femi-diameter of the fphere on which the 

 concave metal is to be ground ; in the fecond column are 

 the proportions of the apertures for thefe feveral lengths ; 

 and in the third column are the proportions of the charges 

 or diameter of the fpheres, on which the convex fuperficies 

 of the eye-glafles are to be ground. 



Suppofe, for example, that a half-foot or fix-mch tele- 

 fcope will magnify diftinftly 30 times with one-inch aper- 

 ture, and that it be required to find the dimenfions of an 

 analogous four-feet telefcope ; then oppofite 4, in column i, 

 we fee 476, in column 2 ; and if 100 at the top of this 

 column be confidered i.oo, then 476 will be 4.76 for the 

 correfponding aperture; alfo 168, in column 3, will be 

 1.68 for the charge, producing a magnifying power of 

 I45 times ; for if the telefcope of fix inches magnify 30 

 times, the focus of its eye-glafs muft be 4^th of an inch, 

 and .^^4 = 1 nearly will be the focal length of the eye- 

 glafs for 4 feet or 48 inches, and confequently this number 

 divided by J will give 145 nearly for its power. " But 

 what the event will really be," fays Newton, " we muft 

 wait to fee determined by experience." And experience, 

 we may add, has fliewn, that a four-feet refleftor, as now 

 made, with the fmall fpeculum alfo concave, will do much 

 more. 



Hadley. — From this time we do not find that much, if 

 any, improvement took place in the manipulation of the 

 metallic fpeculum, till about the year 1719, when Mr. 

 Hadley fucceeded in making two refleAing telefcopes of 

 5 feet 3 inches long ; one of which he prefented to the 

 Royal Society of London, and defcribed in the Philo- 

 fophical Tranfaftions of March and April, 1723. But 

 though Mr. Pound and himfelf made good obfervations on 

 Saturn, Jupiter, and their refpeftive fatellites therewith, 



nothing is faid about the means ufed ia catting, grinding, 

 and polilhing either of the fpecula in queUion. Dr. Ro- 

 bert Smith, in his " Complete Syltem of Optics," has, 

 however, fupplied the omiflion we have juil noticed, and 

 has ftated at coufiderable length how Hadley 's method of 

 proceeding was praftifed fuccefsfuUy by Dr. Bradley and 

 himfelf at Kew, where, after many trials, they produced 

 their firft fpecimen, of 26 inches focal length, i» May, 

 1724. The different procefFes were now made known to 

 Hauklbee, Scarlet, and Hearne, mathematical-inftrument 

 makers in London, who, from that period, continued to 

 raanufafture this inftrument for fale, which before had been 

 made only by and for private individuals. 



We will give here only a fhort detail of the proceffes 

 ufed by Hadky, Smith, and Bradley, in cafting, pohlhing, 

 and figuring their fpecula ; becaufe the modern improve- 

 ments in this curious art claim our more particular notice. 

 Having fixed on the radius of curvature, and propofed dia- 

 meter, of the large fpeculum, two arcs were Itruck with a 

 beam-compafs on two flips of brafs, large enough to re- 

 ceive the curves of the fame radius as that of the fpeculum ; 

 and when the metal was filed away, fo that one of the curves 

 was convex, and the other concave, they were ground 

 within one another with fine emery, till they exaftly fitted 

 each other, and then became gauges for forming the wooden 

 patterns by, which were next made in a lathe, of fomewhat 

 larger dimenfions than were required for the fpeculum, that 

 the rough edges of the cail-metal might be reduced, as the 

 cafe (hould require. From this wooden pattern another of 

 pewter, containing about one-tenth of regulus of antimony, 

 was caft in the ufual way, and turned in the lathe to fit the 

 convex brafs gauge, and to exceed the propofed fpeculum 

 by one-twentieth of an inch in thicknefs, and double that 

 quantity in diameter. The flalks for receiving the pewter 

 pattern, for cafting the fpeculum by, were of iron, and two 

 inches wider, and one inch deeper, than the pattern, in 

 order that the fand might be every where an inch thick ; and 

 the ingates for receiving the fluid metal were fo made as to 

 allow four or five ftreams over the face of tiie fand, which 

 was obtained at Highgate. This fand was neither much 

 wetted, nor rendered too adhefivc by an admixture of clay ; 

 and when the liquid niet.il was poured in, the flaflis were 

 fuffered to remain expofed to the fun, fo as to cool gra- 

 dually, before the metal was taken out, in order that it 

 might neither warp nor crack. With refpeft to the com-- 

 pofition of the metal, abcve 150 different mixtures were 

 tried before one was determined on, which was preferred ' 

 to fome of the reft. One of the inetals was three parts of 

 copper and one and a quarter of tin, which was hard and 

 white, but found to be porous, efpecially if great heat was 

 ufed. Another was fix parts of Hamburgh plate brafs, caft 

 and milled, with one part of tin only ; but the fumes of the 

 calamy itone left fcabrous ftreaks on the lurface. A third 

 metal, therefore, compofed of two parts of the former of 

 thefe compofitions, and one of the latter, was adopted, as 

 being fuperior to either ; but it was found neceflary that 

 the copper and brafs ftiould be melted together feparately, 

 before the tin was melted and poured in. And much im- 

 portance was attached to the immediate pouring out of the 

 whole mixture, as foon as it could be well llirred : for after 

 cooling, lefs heat was neceffar)' to melt the whole mixture, 

 than was fufficient to fufe one of the ingredients alone ; and 

 the lefs heat was ufed in bringing the mafs into a hquid 

 ftate, tiie lefs porous the fpeculum was found to be, wheo 

 polifhed. After the metal thus caft had become cool, it was 

 firft ground ou a common grinding-ftone a little concave, till 



all 



