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contributing to its improvement: and the late discovery of 

 a metallic composition for the mirrors of it, which will bear 

 as high a polish as glass, reflect as mvich light as glass tran- 

 smits, and endure almost equally well, without contracting 

 tarnish, is a farther encouragement to prosecute its improve- 

 ment to perfection. 



Among others, I had formerly, from admiration at its 

 contrivance, bestowed some attention on the mechanism 

 of this instrument: and, as it would have spared me some 

 cxpence of time and trials, if any other person had pre- 

 viously suggested to me the hints, which I am to re- 

 late; I imagine they will be of use to others, in direct- 

 ing or assisting the course of their labour, in the same 

 pursuit. I had also taken some pains, to understand the 

 merits of the different constructions of this telescope: but, 

 as this enquiry ended in a conviction, that the Newto- 

 nian 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 amved 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 mecha- 

 nical fabrication 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. 

 Edwards, in the composition of the specula for telescopes, 

 I 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 



