^^ Speculams of ReJlcBing Tdefcopes, 



after all, is really detrimental. The beft method I have ever found to rough grind the fpe- 

 culum is to grind the furface of it quite bright upon a * common grindftone, made nearly to 

 the figure or focus of the fpeculum by a gage. Take it then to a convex tool, made of lead 

 and tin, or elfe of pewter, and grind the metal upon it with fine emery. This emery, how- 

 ever fine it may be, will break up the metal very much, but we can eafily cure that procefs, 

 as I fliall ihew hereafter. This tool or rough grinder fliould be made of an elliptical fornix 

 and not circular, (for a rcafon I (hall point out hereafter) and of fuch dimenfions that the 

 fhorteft diameter of the ellipfe (hail be equal in breadth to the diameter of the mirror, and 

 the longed diameter of the elliptical tool fhould be to the fliorteft diameter in the proportion 

 of ten to nine accurately, for a reafon to be mentioned hereafter. The manner of working 

 or figuring the metal upon this tool, and indeed upon all the fucceeding tools, is taught in 

 Dr. Smith's Optics, or the before mentioned volume of the Philofophical Tranfa£tions. I 

 fhall refer my reader to thofe publications, as I only mean to give my own improvements- 

 When the metal is brought to a true figure, it muft be taken to a convex cool, formed with 

 fome flones from a place called Edgedon, in Shropftiire, fituated between Ludlow and 

 Bifliop's Caftle. Thefe fkones or hones are of a fine grain, and will eafily cut the metal, 

 and bring it to a fine face. Indeed, the blue hones f ufed in general by the opticians for 

 this purpofe will fcarce touch the metal, and it will be a laborious undertaking to bring the 

 metal to a fine face, fo as to take out all the breaks up from the emery by the common 

 blue hones. By means, however, of the above mentioned flones, they may be eafily 

 ground and truly figured. The bed of ftones (hould be of a circular figure, and but very 

 little larger than the metal intended to be figured upon it, viz. about two-tenths of an inch, 

 but not more for a fpeculum of four or five inches in diameter. If the tool is made con- 

 fidetably larger than the metal, it will grind the metal perpetually into a larger fphere, and 

 by no means of a good figure : if the metal and tool are of the fame fize exadly, the metal 

 will work truly fpherical ; but it is apt to fliorten its focus lefs and lefs, unlefs the metal and 

 tool are worked alternately upwards. It had, therefore, better me made a little J larger 

 than the mirror, when it will not alter its focus. Too much water ftiould not be ufed at a 

 lime upon the hone pavement, or the figure will be very bad, which may eafily be feen by 

 the face of the metal appearing of different degrees of brightnefs in different parts of it. 

 When the metal is brought to a very fine face and figure by the bed of ftones, it is ready to 

 receive a polifh : but before I Qiall give any dire£lions with regard to the manner of polifh- 

 ing it, I muft mention a circumftance or two I had inadvertantly pafled over. The metal 



* The grindftone may eafily be brought to tlie form of the gage by holding the {hai'p end of an iron bar 

 sgainft it while it is turned round, till fo much is worn away from itJ furface as (hall caufe it to take the 

 true curvature of the gage 



f Should any one, however, make ufe of the common blue hones, he rtioald ufe as little water as poffibie 

 when the metal is put upon them, as they will cut much better when barely wet, than if much water is ufed 

 upon their face. 



X About one-twentieth part greater in diameter. 



muit 



