152 



. Agreeable to these positions, I found, in my trials of 

 polishing mirrors in the common way, by straight or round 

 strokes of the mirror, on the polisher, that the operation 



was 



glasses. And I cannot dissent from those, wbo think tliis was the method 

 employed b^' Mr. Short, with such success, in figuring the mirrors of his 

 telescopes; I mean a conduct in the operation, sagaciously adapted to the 

 properties of the pitchy coating, of the polisher. 



It must be obvious to the reader, that none of the remarks or directions, 

 contained in this essay, can be meant to apply directly to the polishing any 

 speculum, whose magnifude is too great, to admit of being moved on a po- 

 lisher, of equal size with itself. Where the friction, and force of cohesion, 

 of such large surfaces in contact, and the weight of the mirror, exceed the 

 motive power that can be employed, a polisher, of less extent than tlio- whole 

 surface of the mirror, must be applied, to traverse, in succession, the seve- 

 ral parts of it; and the motion must be given, not to the mirror, but to 

 the polisher. Instruments of far less enormous magnitude than Doctor Her- 

 scliell's great telescope, are sui generis, and require particular methods of 

 polishing the mirror adapted to their size. For such, no person should pre- 

 suraf to propose any method, which he has not approved in practice : 

 though, as the general principles here laid down, are, with due accommo- 

 dation, applicable to a polibher of any shape or extent of surface; it should 

 sie!en>,' that, if' such great mirrors could be polished by a regular and uniforna 

 motion, their ijolishersuiight be. made such segments or sectors, &c. of the 

 area of each, respective mirror, and of such breadths in different parts; and 

 the furroWs, made in the coating of pitch thereon, of such number, prox- 

 imity, and depth, as to afford, in the tract of the motion of each part, a 

 degree of pressure and friction, reciprocally proportional to the degree of 

 curvature, proper to each concentric zone of the mirror's surface; which would 

 ttend to produce the desired figure, so far as a polisher, covered with pitch, 

 could be nldde ifwtrumental to this purpose. For, though the size and shape 

 of'riie^pdlishef were' to remain unaltered, yet its resi&tence and abrading 

 piiwbr might be consideiably modified, by varying the number and depth of 

 the furrows, made in the pitch which covers it. And the effect of a process, 

 tMSi'tonda«tfid, will be coftimensurate t© the time it is persisted in. 



