92 REFLECTIJfCf TELESCOPE. 



already given, to the -great mirror, in the Newtonian tele- 

 Scope, or to either of the mirrors in the Gregorian, which hap- 

 jpens to be unsuitable to the other one ; I have to observe, 

 that, in my trials, I have found this could be effected on the 

 polisher, without putting the metal to be ground again upon 

 the hones. For if it has, at first, bet-n formed to a toler- 

 ably correct figure, of any species, then a very small re- 

 duction, of the substance of the metal, will produce a suffi- 

 cient alteration of its form. If the change required consists 

 in a diminution of curvature, a continuation of the i)rocess, 

 under the regulation before-mentioned, -will, without any 

 alteration of the polisher, generally, be sufficient to pro- 

 duce it, from the degree of curvature of a circle, to that of 

 theellipse, parabola, or hyperbola, in order; or from any 

 of these, to the others, in succession*. But, if the degree 

 of curvature, already given, is to be increased, and to verge 

 more toward the circle, as the liiinf, (beyond Avhich no con- 

 trivance could carry it,) then the polisher nnist undergo an 

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

 the centre, Icftuncoated with pitch, should be greatly con- 

 traeted ; and, in the case of the little mirror, Avhich has no 

 perforation in it, entirely filled up; save that a small hole, 

 through the polisher, tapering, from the back Of it, up- 

 wards, to its surface, should be left, for the pitch to sink 

 into, when it becomes closed, and too much compressed, at 

 the centre ; and the furrows, in the pitch, gradually deepened 



* Here it may be proper to observe, that, as the curvature is con- 

 gtantly diminishing by the mere continuance of the operation, so the 

 process is not to be pursued any longer, after the polish, and the desired 

 figure, are found to be perfected. And the metal must always be 

 brought to a very fine face, and a correct splierical figure, on the hones, 

 cr on a leaden tool, bedded with the finest washed emery, before the 

 process of polishing commences; because if all scratches, from the 

 grinding,be not previoubly obliterated, the polishing must, in order to 

 cfFnce them, be continued so long, as to diminish the curvature of the 

 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 

 ^ndency, than the former, to diminish the curvature of the mirror 

 and to render it hyperboiicai. And the correction of this, afterward, 

 might require a troublesome alteration of the polisher, or even make 

 it necessary to put the meta! again upon^ the hones; and yet, in the 

 Gregorian telescope, the hyperbolic figure is the proper one, for 

 cither of the mirrors, if that of the other speccium be spherical. 



toward 



