more steady, uniform, and, simple motion, in grinding, may 

 be pursued; which, as it will admit of a less degree of 

 expertness and sagacity in the artificer, will be more com- 

 monly attended with eminent success.* 



The consistence of the pitch is, in this business, an 

 article of the first importance. Soft pitch will give to the 

 polish a higher lustre, and will less expose the face of 

 the mirror to scratches: but, if it be top soft, the mirror 

 will sink in it, like a seal in soft wax; and the figure of 

 .the polisher cannot be preserved, nor the furrows in it, 



* 1 imagine, tiiat a polisher, whose' area is of an oval form, would be better 

 adapted to the formation of a parabolic, than an hyperbolic curvature, in the 

 speculum ; and that the; letter ;will be most correctly formed by a polisher, 

 whose area is nearly circular. For, in order to make the speculum hyperbolic, 

 the longest diameter of the oval polisher must be considerably greater than the 

 shortest oiife) i.e.' than the breadth of th6'' mirror; as will be evident, from a 

 consideration of the circumstances I have endeavoured to explain. And, as 

 the mirror must be carried, by the strokes in polishing, to the extreme verge 

 of the polisher; so, wiien it is to traverse it, in the direction of its longest 

 diameter, it will have its center or vertex removed too far from that of the 

 polishej-,, tp acquire, from it a true conoidal figure. Either, therefore, the 

 face of the polisher should be round; or, if it be oval, it ought to be ren- 

 dered a less eccentric ellipse, by having its shortest diameter greater than 

 that of the mirror, which will allow the extent of the polisher to be reduced, 

 by contracting proportionably its transverse diameter; i. e. it must be brought 

 nearer to a circular figure. For the objection, mentioned by Mr. Edwards, to 

 a round shape of the polisher, when it is to be considerably larger than the 

 mirrw, ;viz. " that it makes the latter grind perpetually into a segment of a 

 " larger sphere, and by no means of a good figure," I apprehend to have 

 chiefly arisen, from an omission, in those who tried it, to make furrows in the 

 pitch, in the proper tract, on the surface of the polisher; which, if it had 

 been done, would have produced, iiot a spherical, but a conoidal figure. 



