REFLECTING TELE5C0PI. fST^ 



permit It to be nicked round with the edge of a file, in cider 

 to break off the prism from the mirror when cast : for thus 

 the heat of the large contiguous body of the prism will keep 

 the neck from congealing; which, if it happened, would stop 

 the liquefied metal, in the prism, from running down into the 

 mirror. And, to prevent this, the prism ought not to form 

 directly a part of the main jet or ingate, by which the metal 

 is poured into the flask; for so the jet would cool sooner than 

 the large mass of the mirror, and bear off the weight of the 

 atmosphere, which ought to press on the fluid metal in the 

 prism underneath, and force it down Into the mirror, to fill up 

 all vacuities in it. Both the prism and the mirror, therefore, 

 ought to be filled by a lateral channel, opening (from the 

 principal ingate) into the top of the prism; which latter should 

 be formed broad and flat, and not taper upward, like a pyra- 

 mid, lest, by cooling where it grows narrow, it might form a 

 solid arch^ and oppose the pressure of the atmosphere. When 

 it is fashioned, as here directed, and made of a bulk equal to 

 a third or fourth part of the mass of the mirror, or even a 

 fifth or sixth part, when the mirrors are of large size, there 

 will ever be found, in the top of the prism, after the metal is 

 cast, a deep pit or cavity, which contained the metal, that 

 had ran down into the mirror, after the outer shell of the 

 mirror, and sides of the prism, had become solid and con- 

 gealed ; and the mirror itself will be found perfect, without 

 any sinking or cavity ; which could only be formed by an 

 injudicious disposition of the jet or appendage, permitting the 

 metal in it to freeze sooner than the whole mass in the mirror, 

 and thus stopping its descent into it. If several mirrors be 

 cast together, in the same flask, there-must be such a separate 

 appendage made to each of them. 



In this manner I have (without a failure in any) cast many The small spe« 



mirrors of different sizes, and sometimes several of them top;e- ^"^""^^ ^^ ^* 

 , . n 1 Ti 11 "isdc out ot a 



ther in one flask. But very small ones, such as the little bar. 



mirrors for Gregorian telescopes, cannot be cast in this man- 

 lier; for their masses being but small, they cool too quickly, 

 to receive any additional infusion of metal; and their outer 

 edges, suddenly forming a solid incompressible arch, the 

 central parts, in contracting towards it on every side, sepa- 

 rate, and ar« rent asunder. And this has happened, even 

 wheft I cast them in brass moulds made red hot : on which 



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