177 



already given, is to be increased, and to verge more to- 

 wards the circle, as the limit, (beyond which no contri- 

 vance could carry it,) then the polisher must undergo an 

 alteration. Its breadth should be diminished; the space, at 

 the center, left uncoated with pitch, should be greatly 

 contracted; and, in the case of the little mirror, which 

 has no perforation in it, entirely filled up; save that a 

 small hole, through the polisher, tapering, from the back 

 of it, upwards, to its surface, should be left, for the pitch 

 to sink into, Avhen it becomes closed, and too much com^ 

 pressed, at the center; and the furrows, in the pitch, gra- 

 dually deepened towards the edges. I believe, that (for 

 the reason before given) the uncoated space, at the cen- 

 ter, ought always to be as much smaller, on every side, 

 than the perforation in the mirror, as the greatest range 

 of the strokes, in polishing, advances the center of the 

 mirror beyond that of the polislier, having the same shape 

 as the polisher itself; and that it ought to be smallest, or 

 no other than as just mentioned, when there is no hole ia 



the mirror. 



1 have, 



must, in order to efface tliem, be continued so long, as to diminish the cur- 

 vature of tlie mirror beyond what is requisite; especially, if the area of the 

 polisher be not of an oval, but of a round shape; which latter has a greater 

 tendency, than the former, to diminish the curvature of the mirror, and to 

 render it hyperbolical. And the correction of this, afteiward, might require a 

 troublesome alteration of the polisher, or even make it necessary to put the 

 metal again upon the hones: and yet, in the Gregorian, telescope, the hyper- 

 bolic figure is the proper one, for either of the mirrors, if that of the other 

 speculum be spherical. 



