i6Q 



6*e uftlftwWm. 'Tn'-^^^rmeHiod-' and' 'with' a' polishei^ of 

 i^und oT ovAl 'shape,' it is indispensably necessary, that 

 there should be furi'ows made in the coating of pitch, (to 

 allow it to subside, in regular' gradation, from the mfiddle 

 W tl^'ifedgfes^) -'by 'ittdteirtin^' it, either in squares, as is 

 tlidallydonei *!>r in (jirciirkr cMHiiel' ';■ both which inust be 

 4<toeWe«d, as thfej" 'l3iecbfAe 'ftTIed 'tip and obliterated; which 

 will ahVftjrs happen soonest in the- middle zone or tract of 

 Ihe polisher, between 'the eelltei* and ottter edgfe, whether 

 «he fun^S^iie 'tfiVchlar x!>i- ISngitudinalv Uiid', if this' be not 

 doftfe, the regularity of curvature would not be preserved 

 in the mirror, or the polisher. But, since th^re is no ob- 

 stacle to the subsidence of the pitch,: near its outer - dge, 

 and its inner edge, when there i^ a tbid space at the cen- 

 tet', I'belileve the furtows bugH't/not to be inade there, but 

 m the intermediate space onjy,, .i-Aiftdlam of opinion feiat 

 it is, from the judicious disposition of these furroAVs, the 

 most correct shape of the mirror is to be acquired; whe- 

 ther the polisher be round or oyal, or the pitch hard or 

 soft: for I found, that, in Mr. Edwards'* method, and Avith 

 pitch, even as hard as he TCcoh^mends, the channels made 

 in it were, towards t|;e .^ud ,pf,tiji,e! operation, nearly ob- 

 literated, in the middle aone of the polisher. But this will 

 not happen so soon, nor so daiigerously, with hard a§ 

 with soft pitch; nor will the correction of the impa,ire4 

 shape of the polisher be so difficult, when it is of an oval, 

 as when of a circular dfea: there being, in the former 

 case^, 1^5 ;pf iiregular surface i^ it, to be ,];p4uced; and a 



more 



