132 



ceivably small is that incorrectness of form, which will 

 produce grievous aberrations of the ra^'s of light, will, I 

 ?ra sure, readily subscribe to the assertion, that ' hoc opua^ 

 * hie labor est.' Methods have indeed been proposed for 

 accomplishing it; but not a single hint given, that I know, 

 of the modus 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 recom- 

 mended. But, on my making the attempt, in the ways proi> 

 posed by Mr. Mudge and by Mr. Edwards, I Avas surprised 

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

 and sometimes (though rarely) a conoidal one, was produced 

 by each: the cause of either being to me then unknown; 

 and disappointment or success appearing to depend on mere 

 accident, and not on the degree of pains and accuracy used 

 in the process. 



At length I began to suspect, that these variations, in 

 the event of the process, (which will be hereafter accounted 

 for,) arose from some property, not adverted to, in the pitch 

 that covered the polishing tool; Avhich material has been 

 generally used for this purpose, of communicating a proper 

 figure, as well as a high polish, to the mirror, since it was 

 first recommended by Sir Isaac Newton; being commonly 

 spread on the polisher, to about the thickness of a crown- 

 jDiece, and then covered with the polishing powder; (the 

 manner of doing Avhich I suppose the reader to be ac- 

 quainted with, as also with what has been made public 



on 



