159 



ing, than others, arising from the difficulty of preserving 

 an uniform pressure during the operation, and, conse- 

 quently, a regular figure of the poUsher. 



Another method, different from that now described, of 

 communicating to mirrors a parabolic form, has been dis- 

 covered by the late Rev. Mr. Edwards, and published in 

 the Nautical Almanack, for the year 1787. He recom- 

 mends, to make the edge of the polisher tlte peri- 

 phery of an ellipse; so that the face or area of it may 

 not be round, but oval: the shortest diameter of the 

 ellipse being equal to that of the mirror; and- its long- 

 est diameter to be to the shortest, as 10 to 9. And he 

 affirms, that a min-or, finished on such a polisher, will 

 prove to be of a parabolic form ; if the process be con- 

 ducted, by employing, througlwut the operation, straight 

 strokes of the mirror, diametrically across the polisher, in 

 every direction. Now, in the method recommended by 

 Mr. Mudge, whatever kind of motion be used, in bring- 

 ing the face of the mirror to a polish, the parabolic form 

 IS directed to be acquired, only by a circular motion in 

 polishing: Mr, Mudge having declared, that the effect of 

 such straight strokes would be, to produce no other than 

 a correct spherical figure in the mirror. Here, then, are 

 opposite motions, and declared' to be productive of con^ 

 trary eflFects, proposed by two very intelligent artists, with 

 a view of promoting the same effect; the only difference 

 being tills, feiat, in the one case, the face of the polisher 



X 2 . is 



