52 REFLECTING TELESCO?E. 



The pitch Agreeable lo these positions, I found, in my irials of po- 



should be a lisJjing mirrors in the common way, by straight or round 

 little harder , /> , . , ,. , , , 



than col^on. strokes or the mirror, on the pohsher, that the operation was 



more easy and successful, when I used pitch of nearly the 

 common consistence, than when I employed such as waj 

 made very hard, by long boiling it, or by the addition of 

 much resin. Such softer pitch will admit more than one ap- 

 plication of the polishing powder, without scratching the 

 metal, or spoiling its previous polish ; by which means, the 

 process will be mofe expeditious. It will instantly accom- 

 modate itself to the successive alterations in the form of the 

 metal ; as this, by wearing down towards its edges, gradually 

 changes from a spherical, to a conoidal shape : and it will 

 promote this effect, by opposing a greater resistance to the 



ly to the polishing any speculum, whose magnitude is too great, 

 to admit of being moved on a polisher, 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 the 

 Whole surface of the mirror, must be applied, to traverse, in suc- 

 cession, the several 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 Herschell's great telescope, are- 

 ■sui generis, and require particular methods of polishing the mir- 

 ror adapted to their size. For such, no person should presume 

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

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

 accommodation, applicable to a polisher of any shape or extent 

 of surface ; it should seem, that, if such great mirrors could be po- 

 lished by a regular and uniform motion, their polishers might be 

 made such segments or sectors, &c. of the area of each respective 

 mirror, and of such breadths in different ports; and the furrows, 

 made in the coating of pitch thereon, of such number, proximity, 

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

 degree of pressure and friction, reciprocally proportional to the 

 degreeofcurvature, proper to each concentric zone of the mirror's 

 surface ; which would tend to produce the desired figure, so-far as 

 a polisher, covered with pitch, could be made instrumental to this 

 purpose. For though the size and shape of the polisher were to: 

 remain unaltered, yet its resistence and abrading power might be 

 considerably modified, by varying the number and depth of the 

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

 process, thus conducted, will be commensurate to the lime it is 

 persisted in. 



