REFLECTING TELE3C0PB. 51 



Among others, I had formerly, ^ora admiration at itscon"- introduction 



trivance, bestowed some attention on the mechanism of this '^^^^^^"^ ^"^ 



, . , , , , Structure, 



instrument: and, as it would have spared me some expence 



of time and trials, if any other person had previously suggested 

 to me the hints, which 1 am to relate ; I imagine they will be 

 of use to others, in directing or assiting the course of their la- 

 bour, in the same, pursuit. I bad also taken some pains, to 

 understand the merits of the different constructions of this te- 

 lescope: but, as this inquiry ended in a conviction, that the 

 Newtonian form of it is the most perfect that can be hoped 

 for ; (it being the nature of its great author to persevere in his 

 researches, till he had arrived at a complete solution of his 

 doubts, and comprehension of the subject;) so I have only to 

 report what resulted from my experience in the mechanical fa- 

 brication of it, as to the method of casting the mirrors, and 

 communicating to them the proper figure. 



Before I had heard of the improvements of the Rev. Mr. On the nature 

 Edwards, in the composition of the specula for telescopes, I oftheMetaL 

 had made many experiments myself with that view ; which 

 lead me to give full credit to his report of the superior excel- 

 lence of that composition which he recommends : because I 

 had found, that the qualities of hardness, whiteness, and indis- 

 position to contract tarnish, necessary to, a speculum, could 

 not, by any admixture that I could hit upon, be produced, un- 

 less the metal were so highly saturated with tin, as to be ex- 

 cessively brittle ; and because I found that this brittleness, 

 however Inconvenient in some respects, was necessary to ren- 

 der it susceptible of the highest polish : for no metal yet 

 known, except steel, (which, from its disposition to rust, is un- 

 fit for this purpose,) will take as high a polish as glass will, un- 

 less it be more brittle than glass. And indeed this properly is 

 common to all substances which we know, that are capable of 

 such polish : they must be very hard, and, as such, brittle ; for 

 the polishing powder employed would stick and bed itself in 

 any soft metal, instead of cutting and polishing it. 



From the result of my trials, I contented myself with the Silver rencJers 

 composition mentioned hereafter, as being in every respect suf- ^t soft ; but the 

 ficientfor the purpose, and inferior to none in whiteness, lustre, extended*^ "°'' 

 and exemption from tarnish : for, as to the addition of silver, I 

 found that, when used even in a very small quantity, it had aa 

 extraordinary property of rendering the metal so soft, that I was 



