The next step is to put the reflector, thus raised nearly to its true form, into an iron stool, where a small hole being drilled in its vertex, a circle is described from this point with a beam-compass, so as to cut the paraboloïd to the proposed size.
Fig. 102.
Fig. 103.
Fig. 104.
Fig. 105.
The reflector is next hard-hammered all over (or planished, as it is technically termed) on the bright steel-head a ([fig. 103]), with the planishing hammer ([fig. 102]); and to facilitate working, the reflector is slung in a flexible frame SS, and counterbalanced by a weight w, hanging by a cord over the pulleys p p. When the reflector is all planished over, the next process is the smoothing, which is done on the steel-head a, with a lighter hammer ([fig. 104] ), muffled with fine parchment at each end. After it is smoothed comes the finishing, or what is called the filling up to the mould. This is a tedious process, and the workman requires continually to have recourse to the marble table at M, on which he lays the reflector, as shewn in [fig. 105], and applies to successive portions of its surface the mould g n, which has a needle-point centred at n, in the small hole drilled in the vertex. During this examination, he marks with a fine slate-pencil those portions of the reflector which do not meet the mould g n. The parts, so marked, are gently gone over with the muffled hammer, until every point touches the mould. This last process requires great caution; for, if any part of the surface be raised above the gauge, it is hardly possible to remedy it. Such a mistake, indeed, can only be corrected by annealing the reflector afresh, and bringing it back to the true form with the mallet; but reflectors so cobbled are never good. The table M ([fig. 105]) rests on a square box C, in which the tools and moulds are kept.