3S REFLECTING TELESCOPE. 



account, 1 have been obliged to form them out of pieces of the 

 metal, cast In long thin ingots or bars ; which, by nicking 

 them across with a file, could be easily broken into square 

 pieces, whose corners could be taken off, and rounded in the 

 same manner. 

 Reference to ^ ^^ "ot repeat the other precautions to be observed in this 

 Edward's Trea- process, which have been already so well and sagaciously 

 Tournal ^quarto described by the Rev. Mr. Edwards : but the circumstances 

 series, vol, V. above mentioned, a prudent attention to which, is, in my 

 opinion, essentially necessary to the success of it, are not to 

 be collected from any directions published on the subject that 

 are known to me. And though particular artists may, by 

 large experience, arrive at a sufficient knowledge in this 

 matter, for their own practice; yet, to render that knowledge 

 general, and to contribute, as far as I could, to the improve- 

 ment of this instrument in any hands, being the design of this 

 essay, I thought it necessary to state the above particulars 

 fully ; though I doabt not that these, as well as other matters 

 of moment in the operation, are known to many, who chuse 

 not to make them public. Thus the great skill, in the con- 

 struction of the telescope, acquired by Mr. Short, seems not 

 to have been transmitted to any successor. 

 Figuration of I come now to speak of the most difficult part of the me- 

 che mirrors. chanism of this instrument, that of communicating a proper 

 figure to the mirrors ; on which depends the powers of the 

 telescope, when its dimensions are given : for the manner of 

 polishing them, to the highest degree of lustre, has been al- 

 ready well understood and described. They who have tried 

 this part of the w^ork, and know how inconceivably small i^ 

 that incorrectness of form, which will produce grievous aber- 

 rations of the rays of light, will, I am sure, readily subscribe 

 to the assertion, that ' hoc opus, hie labor est.' Methods have 

 indeed been proposed for accomplishing it; but not a single 

 hint given, that I know, of the tnpdiis operandi, or the grounds 

 of these methods: insomuch, that, when I first tried to polish 

 mirrors, I had no idea why any figure of them, different from 

 that of a sphere, should result from the modes of polishing 

 recommended. But, on my making the attempt, in the ways 

 proposed by Mr. Mudge and by Mr. Edwards, I was surprised 

 to find, that sometimes a spheroidal or other irregular figure, 

 jBind sometimes (though rarely) a conoidal one, was produce^ 



