A SILVERED GLASS TELESCOPE. 



23 



Fig. 23. 



Local Polisher. 



Fig. 24. 



miiTors which are as perfect as can be, and yet only requiring a short timr. It 

 IS the correction of a snrfecc by local r<>touches. In the account pnl)lisl,c(l I,y M. 

 Foucault, it appears that he is in France the inventor of this improvement. 



The mode of practising the retouches is as follows: Several disks of wood, as o. 

 Fig. 23, varying from 8 inches to i an incli in diameter, are to be provided, and 

 covered with pitch or rosin of the usual harchicss, in squares 

 as at r, on one side.' On tlie other a low cylindrical handle 

 h: is to be fixed. The mirror «, Fig. 24, having been fined 

 with the succession of emeries l)efore described, is laid face 

 upward on several folds of l)lanket, arranged upon a circular 

 table, screwed to an isolated post iu the centre of the apart- 

 ment, which permits the oi^erator to move completely roiuul it. An ordinary barrel 

 has generally supplied the place of the post, the head e, Fig. 24, serving for the 

 circular tabl(>, and the rim h preventing the mirror sliding 

 off. The other end is fastened to the floor by four elects tJ <V . 



The large polisher is first moved over the surface in straight 

 strokes upon every cliord, and a moderate pressure is ex- 

 erted. As soon as the mirror is at all brightened, perhaps in 

 five minutc-s, the operation is to be suspended, and an ex- 

 amination at the centre of curvature made. By carefully 

 turning round, the best diameter for support is to be found, 

 and marked with a rat-tail fih^ ou the edge, and then the 

 curve of the mirror ascertained. If it is nearly spherical, 

 as will be the case if the grinding has been conducted with 

 care and irregular lieating avoided, it is to ho replaced on the blanketed support, and 

 the previous action kept up until a fine polish, free from dots like stippling, is 

 attained. This stage should or-cu[)y three or four luiurs. Another examination 

 sliould reveal the same appearances as the preceding. It is next necessary to 

 lengthen the radius of curvature of the edge zones, or what is much better shorten 

 that of the centre, so as to convert the section curve into a parabola. This is 

 accomplished by straight strokes across every diameter of the face, at first with a 

 4 inch, then with a 6 inch, and finally with the 8 inch polisher. Examinations 

 must, however, be made every five or ten minutes, to determine how nuuh lateral 

 departure from a direct diametrical stroke is necessary, to render the cur\'e uniform 

 out to tlie edge. Care must be taken always to warm the polisher, either in front 

 of a fire or over a spirit lamp, before using it. 



Perhaps the most striking feature in this operation is that the mirror ]>resents 

 continually a curve of revolution, and is not diversified with imdulations like a 

 ruffle. By walking steadily round the support, on the top of wliich the mirror is 

 placed, there seems to be no tendency for such irregidarities to arise. 



If the correction for spherical aberration sliould have proceeded too fiir, and 

 the mirror become hyperbolic, the splun-e can be recovered by working a succession 



Section of Optician's Post. 



• M. ront-aiilt used plano-convex lenses of gliiss, of a railius of eiirvalniT sliirlitlv le^s iLau that 

 of the mirror, and c;ovcre<l with paiirr on ihe eonve.v faeu. 



