KEFLECTING TELESCOPE. §3 



metal, and greater friction towards its extremities, when its 

 previous disposition on Lh.e polisher has been judiciously pro- 

 vided, in the manner before explained. 



But, to fulfil these intentions effectually, a certain kind of Description of 

 motion, of the mirror on the polisher, must be carefully ob- stroi^e^mosf. 

 served, during the operation : for, as the softer pitch will proper for giv 

 continually yield, and sink under the pressure of the metal j '"S '^® Hw^ 

 so, the form of the polisher, degenerating in every stroke, 

 Siust be recovered, and preserved correct. According to the 

 principles before laid down, the face of the polisher must be 

 considerably larger than that of the metal, in order to afford 

 a greater resistance to the speculum, tovs'ards its extremities : 

 so that, as the metal covers only a part of the polisher, if the 

 former were to be confined in its motion, the pitch, sinking 

 under it, would expand itself laterally, and become heaped 

 up suddenly, around the tract of the mirror^s pressure ; 

 which must, therefore, to obviate this, be so conducted, as 

 to traverse, in quick and regular succession, every part of 

 the polisher, in order to recover the regularity of its figure as 

 fast as it becomes vitiated. And this is effected in two ways: 

 either by enlarged circular strokes of the metal, brought 

 considerably beyond the edges of the polisher, in order to 

 repress, towards the center, the pitch, which had become 

 raised near its edges, or by straight diametrical strokes, across 

 its surface, in every direction successively : either of which 

 will tend to preserve the figure of the polisher, and, conse- 

 quently, of the mirror, nearly spherical. As, however, a 

 spherical figure is not that which is ultimately intended, so 

 these modes of conducting the process are to be pursued 

 only till the mirror has acquired a sufficient polish, and a figure 

 nearly spherical : and then, in order to give it a parabolic or 

 hyperbolic shape, the motion of the mirror, on the polisher, 

 should be such, as that the center of it may describe a spiral 

 line round the center of the polisher, by enlarging the cir- 

 cular strokes, till the edge of the mirror arrives at the edge 

 of the polisher ; and then contracting the motion gra« 

 dually, till the mirror returns to the center, in the same spiral 

 course. By which means, any sudden and irregular eleva- 

 tion of the pitch, beyond the place of the mirror, will be 

 prevented : while, at the same time, it will become regularly 

 elevated, from the outer edge, in the form of a conoid, and 



£3 



