161 



der, that modes ef conducting it, seemingly so dissimilar, 

 tend to the same effect; and perceive, that the contra- 

 riety is not real, but merely apparent.* 



For, in either method, it is not the direction of the 

 motion employed, nor the shape of the area of thfe po- 

 lisher, which, : in realirty, 'pi-b-duces a' conoidal form m the- 

 mirrorr ' but a gradiial alteratioti iti the curvature of the 

 face of the polisher, by the yielding of the pitch, under 

 the pressure. ' And, therefore, when any part of the area 

 of the polisher, whether it be' round or oval, is^ more ex- 

 tended than that bi' the mih'or ; the 'pitch, moving late- 

 rally, will bfecome elevated, and its curvature lessened, in 

 that part. So that,' in a polisher of dval shape, whose 

 conjugate diameter is eiqiial to that of the mirror, the 

 pitch witl ascend and accumulate, in the part, which lies 

 without the circular' arfeal'df the^'miri'dr, 'inidribeci lit • the 

 ellipse. The extremities of the mirror will, tTiereforei,' be 

 woi-n down, when ea;ch part of theni is inade in rotation, 



' • ■- . :- ■ --q.ijii /• ■ •;■■' by 



lO'i vnr 

 * In the npethods of figuring the mirrpfs, pqWishetj by. Mr. Mudge, and 

 by Mr. Edwards, it is stated by Mr, Mudge, that he frequently, during the 

 process, applied to the' pblisHer a conCave tool, which he call% a bruisei";: 

 by which, he miast have preserved, or recovered, the figure of the polisher, 

 and, consequentl3', of the mirror, that otherwise must haycj become. , vitiated, 

 by the unequal resistance of the pitch; and Mr., Edwards ipade furrows in 

 the coating of pitch, "on his polishers. It fs to these circiinistances, and not 

 to the direction of the motion employed, or the elliptic atea of the po- 

 lisher, that, 1 can think, was owing the success, attendant on- their mqthodsr. 

 the bruiser being necessary, to supply the defect of furrows in the pitch j 

 and the oval form notf' essential, Svhen there were such, duly disposed, and 

 also- the polishec of proper size, &c. as here directed. 



