4(}0 Cprnpojition and Fahrication of tkt 



IV. 



Dirc&ions for making the hjl Compojition for the Metals of reJleBing Telefcopes, and ifee 

 Method ofCaJling, Grinding, Polijhing, and giving the great Speculum the true parabolic 

 Figure. By the Rev. John Edwards, B. A*. 



T. 



HE methods in general ufed for cafting, grinding, and poliflilng the metals for re- 

 fleding telefcopes being well known to workmen, and having been treated of in the moft 

 full and fatisfaftory manner in Dr. Smith's optics, and alfo by Mr. Mudge, in the Philo- 

 fophical Tranfaftions, Vol. LXVII. Part. I. I (hall not dwell upon thefe points, but Ihall 

 add fuch dire£l:ions and obfervations of my own, as I have found by experience to anfwer 

 much better than the method taught by thofe writers. Some telefcopes conftrudted by me 

 have been tried by the Rev. Dr. Malkelyne, Aftronomer Royal, and found very greatly to 

 excel in brightnefs t, and to equal in other refpe£ts telefcopes of the fame fize, con- 

 ftru£ted by the beft artifts in London. 



Of the bejl Compofitlon for refleEllng Specula. 



That I may not be tedious upon this point, it may be nccefiary to a'cquaint my reader, 

 that I have made experiments upon the following metals and femi-metals, in order to dif- 

 cover a compofition for a fpeculum which fhould refle£t the greatefl: quantity of light, and 

 confequently be capable of receiving the fined polifh. I combined them in feveral pro- 

 portions, and ground and polilhed them. The metals and femi-metals I tried were filver, 

 pLftina, iron, copper, brafs, lead and tin, crude antimony, regulus of antimony, martial 

 regulus of antimony, arfenic, bifmuth, zinc, and antimony combined with cawk-flone %. 

 Having tried many compofitions of them, (fee the Appendix) I found that 32 ounces of 

 copper, with 15 or 16 ounces of grain tin (according to the purity of the copper) with the 

 addition of a little § brafs and arfenic ; viz. one ounce of each to the above proportion of 

 copper and tin, will form a metal, capable when polilhed in a proper manner, of refle£ling 

 much more light than any other metal, that has as yet been offered to the public. When I 



• From the Nautical Almanac for 1787, inferted at the requeft and fuggeftion of a correfpondent, who 

 obferves, that the almanac for that year has become very fcarce in confeq^ience of the demand for this 

 treatife, which is not elfewhere to be found.---N. 



•}• Mr. Edwards' telefcopes fliew a white objeft perfe6Uy wliite, and all objefls of their natural colours ; 



very different fiom common reflefting telefcopes, which give a dingy copperifli appearance to objefts. I 



found by a careful experiment, that they ihewobjefls as bright as a treble objeft glafs achromatic telefcope, 



both being put under equal circumftances of areas of the aperture of the object metal and objeft glafs, and 



equal magnifying powers ; whereas the diameter of the aperture of a common reflefting telefcope muft be 



to that of an achromatic telefcope as 8 to 5, to produce an equal effeiSl. 



Nevil Maskelyne. 



J See a moft curious experiment upon cawk ftone and antimony in the Philofophical Trnnfaftions, 



No. ex. 



§ If one (»ince of fiirer be added to this compofition, the metal will be much better and whiter. 



fay 



