ueflecting telescope. 51 



a metallic speculum, though it would to glass, crystal, or 

 jewels ; because no metal can be cast, perfectly free from small 

 pores : and any elastic substance, if employed to polish it, ^ 

 would insinuate itself, together with the polishing powder, 

 into these pores, and wear down their edges in such a manner, 

 as to convert every pore into a long furrow or cavity: which 

 would occasion the destruction of the whole surface of the 

 metal, as was truly observed by Sir Isaac Newton. And thus 

 it appears, that, to make the pitch too hard and refractory, 

 would be to destroy every property in it, which renders it 

 ehgible in this operation. 



If the positions, before statedi be well founded. It seems to -j-he polisher 

 follow, that the desired change in the mirror, from a spheri- must change 

 cal to a conoidal figure, can only be effected, by a change in j^a^ of th^mlr, 

 the shape of the polisher, gradually accommodating itself to the ror changes. 

 alteration, produced in that of the mirrOr^ during the process 

 of polishing. Nor, indeed, can it well be conceived, how 

 the mirror could alter Its spherical form, if that of the poHsher 

 remained unaltered; for a conoid could never, in the usual way, 

 and without a partial separation of the surfaces in contact, be 

 polished on a segment of a sphere, nor even on that of a conoid, 

 if, during the friction of their surfaces, the center, or vertex 

 of the one, were to be moved to any considerable distance 

 from that of the other. So that the strokes, in polishing, must 

 never ultimately be carried so far as to remove the center of 

 the mirror to too great a distance from that of the polisher; 

 even though its surface were so hard, as to preserve its figure 

 unaltered by the pressure of the mirror.* 



* For the several reasons above mentioned, I am inclined to 

 think, it will be very difiicult to discover a method, different 

 from that here explained, of communicating, at t)ie same time, a 

 perfect figure and polish to a speculum. It is plain, that New- 

 ton could think of no better ; though 1 imagine that, in this in- 

 stance, he tried his inventive powers with those of Des Cartes, 

 who had published a method (in theory elegantly geometrical) 

 of figuring optic glasses. And I cannot dissent from those, who 

 think this was the method employed by Mr. Short, with such 

 success, in figuring the mirrors of his telescopes ; I mean a con- 

 duct 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 essav, can be meant to aoply direct- 



£. 2 



