A SILVERED GLASS TELESCOPE. 



23 



Fig. 23. 



Local Polisher. 



Fig. 24. 



mirrors which arc as perfect as can be, and yet only requiring a short time. It 

 is the correction of a surface by local retouches. In the account published by 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 a, 

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

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

 as at <;, on one side. 1 On the other a low cylindrical handle 

 l\ is to be fixed. The mirror , Fig. 24, having been fined 

 with the succession of emeries before described, is laid face 

 upward on several folds of blanket, arranged upon a circular 

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

 ment, which permits the operator to move completely round it. An ordinary barrel 

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

 circular table, and the rim It preventing the mirror sliding 

 off. The other end is fastened to the floor by four cleets d d'. 



The large polisher is first moved over the surface in straight 

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

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

 five minutes, 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 file on 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 heating avoided, it is to be 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 occupy three or four hours. Another examination 

 should 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 much lateral 

 departure from a direct diametrical stroke is necessary, to render the curve uniform 

 out to the 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 presents 

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

 ruflio. By walking steadily round the support, on the top of which the mirror is 

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



If the correction for spherical aberration should have proceeded too far, and 

 the mirror become hyperbolic, the sphere can be recovered by working a succession 



Section of Optician's Post. 



1 M. Foucault used plano-convex lenses of glass, of a radius of curvature slightly less than that 

 of the mirror, and covered with paper on the convex face. 



