to the Specutums of ReJleBing Tdefcopes. 527 



you may »dd a little more water, to carry it lower down to what degree you pleafe. When 

 the colcothar of vitriol has been wrought dry three or four times, it will acquire a black 

 colour, and will be low enough, or fufficiently fine, to give an exquifite luftre. This le- 

 vigated colcothar of vitriol I put in a fmall phial, and pour fome water upon it, and after- 

 wards I ufe it for polifhing the metals, in the fame manner that waflied putty is aWays 

 ^ direfted to be made ufe of for that purpofe. I always put on a large quantity of walhed 

 colcothar of vitriol at once, fo as to faturate the pitch, and form a fine coating of the col- 

 cothar, and very rarely make ufe of a fecond application. If a fccond or third application 

 of colcothar fliould be found necelTary to bring the metal to a high luftre, or to take out 

 any fcratches upon its face, ufe it very fparingly, or you will deftroy the polifli you have 

 already attained. When the metal is nearly poliflied, it will always generate fome black 

 mud upon the furface of the mirror, and alfo upon the tool. , Wipe it now away from the 

 face of the metal with fome very foft wafh leather; though if too much of this mud be 

 taken away, it will not polilh fo well. Indeed, a little experience in thefe matters will 

 better fuffice, than a volume written upon the fubjedl. 



In regard to the parabolic figure to be given to the metal, no particular caution is re- 

 quired in the polifhing ; the elliptical tool will always caufe the fpeculum to work into an 

 accurate parabolical figure, fuppofing the tranfverfe and conjugate diameters bear the true 

 proportion to each other, and the metal is not too thick to prevent it always from adhering 

 firmly and uniformly to the polifher. Should the pitch prove too foft, it will give way 

 and alter the figure a little. This circumftance will render the figure of the mirror fome- 

 times a fmall degree (hort of the parabola, and fometimes a very little beyond It % but by 

 a little perfeverance the correal figure is very eafily acquired. I could very eafily give the 

 reader the reafon, why an elliptical tool of a proper proportion will always give a parabolical 

 figure ; and if the tranfverfe diameter is increafed, it will then always give an hyperbolical 

 figure; but as I am writing upon the practical part of making refleding telefcopes, and 

 not the theory, I will not oiFend his patience. To convince any one of the certainty of 

 my aflejtions, let him polifh a metal of 2 i inches diameter, and 94- inches focus, upon an 

 elliptical tod, whofe diameters are 2^ and 3 inches ; and I can aflert, he will always find 

 the metal, when polilhed, (if it is not too thick) beyond the parabola, or it will always 

 prove hyperbolical. If he poliflies it upon a circular tool in the common way, with crofs 

 ftrokes in every direftion polTible, ufing firft a few round ftrokes every time he changes 

 his pofit'on, he will find it will always prove fpherical, and confequently^arf of the pa- 

 rabola. A very little experience in thefe matters, will convince any one of the eafe and 

 certainty of giving the great fpeculum a parabolic figure, by polifliing it in a common 

 manner only withcrofs ftrokes in every poflible dire£lion, upon an elliptical tod of the pro- 

 per dtraenfions, in which for common foci and apertures, viz. 2i to py focus, or 3,8 

 inches in diameter to 18 inches focus, the diameters fliould be 10109. The fliorteft 

 diameter of the ellipfe being accurately the fame as the diameter of the metal, and the 

 longeft diameter of the ellipfe to the fliorteft diameter, as 10 to 9. 



Ludlow, July 19, 1 78 1. ' JOHN EDWARDS. 



3 Y 2 . APPENDIX. 



