SPECULUM. 



powers, and prices, of reflefting telefcopes, conftrucled by 

 him in the Gregorian form ; which, not being generally 



known, we (hall fubjoin, by way of effacing the blot on 



his memory. 



In this table Mr. Short always overrated the higheji 

 power of his telefcopes, according to the authority of the 

 Rev. John Edwards, of whom we (hall have occafion to 

 fpeak hereafter ; and to this circumilance alone, we are 

 alTured by an ingenious and candid optician, it is to be 

 afcribed, that all following opticians have been obliged, by 

 way of fatisfying the expeftations of their cuftomers, to 

 overrate the higheft power of their telefcopes, in order to 

 obtain a price adequate to their refpeftive values. Experi- 

 mental meafurements, indeed, foon detect this falfe repre- 

 fentation ; and it would be to the credit of opticians, if 

 they would agree to fix a true meafure to each power of 

 their telefcopes, and to fell them, notwithftanding, at their 

 accuttomed prices : for then their purchafers would not ex- 

 perience difappointment. 



Short made two or three Gregorian telefcopes of 1 8 inches 

 focus, with 4.5 inches aperture, and power 170; and half 

 a dozen of 24 inches focus, with 6 inches aperture, with 

 the ufual magnifying powers ; but one of a CalTegrain form, 

 with a convex fmall fpeculum, of the fame dimenfions, in- 

 tended to have a power of 355, was not diltinft, and would 

 bear only a power of 23 1. 



Mudge. — In the year 1 777, Mr. John Mudge produced 

 a paper to the Royal Society, which is printed in their 

 67th volume, entitled " Direftions for making the bed 

 Compofition for the Metals of refleding Telefcopes ; to- 

 gether with a Defcription of the Procefs of grinding, po- 

 li(hing, and giving the great Speculum the true parabolic 

 Curve." It will be fufficient for our prefent purpofe, if 

 we (hew in what refpefts Mr. Mudge's metal difiFers from 

 that which we have juft defcribed, and in what his improve- 

 ments in calling confift. In the firft place, Mr. Mude^e 

 attempted to afcertain what metals, and proportions of 

 thofe, will produce a compofition that has the three requi- 

 fites of extreme hardnefs, whitenefs, and compaftiiefs, or 

 exemption from porofity. The firft two quaUties, after 

 many experiments, he found were united in a mixture of 

 14! ounces of grain-tin with two pounds of good Swedi(h 

 copper ; but the quahty of compaftnefs was not to be ac- 

 quired from the mixture fimply confidered : it was pro- 

 duced principally from the manner in which the mixture 

 was made. It had been ufual to melt the copper firft, and 

 then to add the tin, and to pour the liquid mixture, with 

 the fcoria firft taken o(F, into the fla(ks, to form the fpe- 

 culum in the firft melting ; but the metal generally turned 

 out to be porous, when poliftied. Mr. Mudge, on con- 



fidering that putty is tin calcined, began to think, that part 

 of the tin melted in a high heat might be converted into 

 putty, and that the pores might arife from this caufe ; he, 

 therefore, poured out the liquid compound to form an in- 

 got, which would fufe wi:h a lower heat than the copper 

 alone required ; and on ufing this metal a fecond time, fufed 

 with a moderate heat, it was generally, if not always, 

 found compaft. Thus melting a fecond, or even a third 

 time, proved to be a remedy againft porofity ; and it was 

 afterwards found, that if an ounce of the tin was referved 

 to be put into the mixture at the final melting, the bril- 

 liancy of the metal was thereby increafed. If a fpoonful 

 of charcoal duft be thrown into the crucible, before the 

 metal be ftirred with a wooden fpatula, and poured into the 

 moulds, Mr. Mudge fays that the metal will be found to 

 have a cleaner furface, than when no fuch duft is ufed. 

 For Mr. Mudge's method of grinding, poliihing, and 

 figuring his fpecula, fee our article Gkixding. 



Edwards.— In the Nautical Almanac of the year 1787 

 is printed a memoir, entitled " DireAions for making the 

 beft Compofition for the Metals of reflefting Telefcopes, 

 and the Method of cafting, grinding, polifhing, and giving 

 the great Speculum the true parabolic Figure," by the 

 Rev. John Edwards, B.A., of Ludlow. Thefe direftions 

 having been confidered an improvement on Mr. Mudge's, 

 in feveral refpefts, were pubhfhed firft in a pamphlet, 

 which was foon bought up by the trade ; but at the requeft 

 of the compofer of the prefent article, they were reprinted 

 in Nicholfon's Philofophical Journal, vol. iii. 8vo. Series, 

 p. 490, et. feq., in the year 1800; but with the omilfion 

 of a portion of the appendix. 



After what we have already faid on this fubjeft under our 

 article GniNDtNG, as well as in the preceding part of our 

 prefent article, it will not be neceffary to give the whole of 

 Mr. Edwards's memoir, but to dwell more particularly on 

 thofe methods of proceeding which are peculiarly his own, 

 and which conftitute his acknowledged improvements. The 

 firft objeft that Mr. Edwards had in view was, like that of 

 his predecelTors, to afcertain, by a number of trials of va- 

 rious mixtures, made from the metals and femi-metals, what 

 compofition is, on the whole, to be preferred : for which pur- 

 pofe he tried filver, platina, iron, copper, brafs, lead, and 

 tin, crude antimony, regulus of antimony, martial rcgulus 

 of antimony, arfenic, bifmuth, zinc, and antimony com- 

 bined with cawk-ftone ; and in the appendix to his memoir 

 he gave the refult of 71 mixtures, which we will here 

 9 tranfcribcj 



