The grinding and polishing of specula for reflecting telescopes requires the greatest possible amount of accuracy and care, and is by far the most difficult of all the processes of grinding and polishing for the production of form. The perfection of the refracting telescope is in great measure limited by the difficulty of grinding and polishing the lenses to the correct spherical figure, but an amount of error that would be quite passable in the best lenses, would be altogether inadmissible in the specula of large reflecting telescopes, consequently a very high degree of accuracy of form is essential, and at the same time a high polish is of necessity required to produce a reflecting surface. The ordinary difficulties of producing very accurate and highly finished surfaces are also increased by the untractable nature of the alloy of which speculums are formed.
Some remarks on the composition of speculum metal have been offered at page 270 of the first volume of this work, and other particulars on the method of casting specula are given in the foot note, pages 371-2, and also in the Appendix, note F, page 462. This interesting subject will be here followed up by some observations on the mode by which the castings, whether of small or large size, are ground and polished to adapt them to the telescope.
The process of grinding and polishing specula of small size by hand will be first described, and the application of machinery to the figuration of specula of large and medium sizes, will be afterwards adverted to. The hand process is subject to small variations in the practice of different individuals, but these variations are made principally in matters of detail that do not affect the general method, or materially influence the result, and are therefore omitted from the description to avoid unnecessary complication.
In grinding specula by hand, the same general method of manipulation is adopted as for grinding the best concave lenses, that is convex tools formed of the same curvature as the required specula are fixed upon a vertical post, and the work is rubbed upon the tool with circular and elliptical strokes in all directions, while the operator continually walks around the post to change the angle of the strokes.
The speculum after having been carefully annealed, is attached by the cement made of pitch and wood-ashes to a metal back, to support it during the working, and serve for the attachment of the wooden handle. The back is made from two-thirds to three-fourths of the diameter of the speculum, and its face is made concave to exactly fit the convex side of the speculum. The back has in the center a screw by which it can be mounted on a lathe to make the edge of the speculum circular, first by holding a fine file to the revolving edge, and afterwards either a metal grinder supplied with emery, or a piece of blue polishing stone.
A pair of brass templates are prepared to the exact radius required in the speculum, in the same manner as for lenses; but they are more carefully fitted together. The rough face of the speculum left from casting is sometimes removed on a common grindstone, turned as described on page 1108-9, nearly to fit the concave template. At other times the speculum is rough ground with coarse emery, on an iron or pewter tool fixed on the post. This grinding is continued until any holes in the surface of the casting are removed, and the face is made quite bright.
The smooth grinding is next effected with fine emery upon a convex pewter tool, turned exactly to fit the template. This tool is usually made circular and slightly larger than the speculum. But the Rev. J. Edwards recommends that the form should be elliptical, in order that the same tool may serve for the foundation of the polisher; this is however not very important. The smooth grinding is continued with fine emery until the face of the speculum is brought very nearly to the true curve. But however fine the emery may be, it is very liable to break up the surface of speculum metal into small holes, notwithstanding the greatest care, and therefore as soon as the speculum has been brought to a nearly true figure, the smooth grinding is discontinued, to avoid the risk of depreciating the surface.
The face of the speculum is next very carefully ground to a fine surface, and as true a figure as possible, upon a bed of hones which is made of small pieces of either blue polishing stone, or Water-of-Ayr stone, cemented upon a pewter tool with pitch and wood ashes. The stones should be carefully selected as homogeneous as possible, and sawn into blocks about three quarters of an inch cube, the tool is warmed to ensure the hold of the cement, which is then melted and spread uniformly over the surface, the stone cubes are carefully arranged upon the tool in straight lines about one-eighth of an inch asunder, and if the stones are of unequal hardness, it is necessary to scatter the hard and soft pieces as equally as possible, in order that the bed of hones may wear uniformly, the stones should not however differ materially either in hardness or grain, otherwise the correct figure of the speculum will not be attained.
After the stones are arranged in their places and slightly pressed into the cement, the interstices are filled with melted cement to within about a quarter of an inch of the face. The general surface of the bed of hones is then turned very carefully to the curve of the template; to avoid accident it may be roughed out with the ordinary sliding rest into the form of a shallow cone, as the convexity required is very slight.
The bed of hones is used with very little water, and cuts smoothly, so that the roughness left by the emery may be entirely removed, and the speculum brought to a very good surface. At the first commencement it appears to act very slowly, but after the principal prominences are reduced it acts more quickly. Great importance in the figuration of the speculum is attached to the proper management of the bed of hones, which is applied with circular and elliptical strokes, exactly the same as the other tools. The Rev. Mr. Edwards says, the bed of hones should be of a circular figure, and but very little larger than the metal intended to be figured upon it. "If the tool is made considerably larger than the metal, it will grind the metal perpetually into a larger sphere, and by no means of a good figure, if the metal and tool are of the same size exactly, the metal will work truly spherical, but it is apt to shorten its focus less and less, unless the metal and tool are worked alternately upwards,-it had therefore better be made about one-twentieth part larger than the mirror, when it will not alter its focus."