The convex mirror is comparatively of no importance, and tin formula) for It may be :easily derived from those for the concave refer fora multitude of minute details to Holtzapffers,',31echanical Mani pulation,' to papers in the proceedings of the British Association, 1843, 1850. to Transactions of the Royal Astronomical Society,' to the ' Philosophical Transactions for 1850,and also to the article Speculum in Nichol** Cyclopsedia of the Physical Sciences; 1860. All wo can do, in this place, is to indicate the nature of the problem to be solved, and briefly to state some of the means adopted for Its solution.
And first air to the material. Speculum motel Is an alley of tie and copper, which according to Lord Roane should be a definite atomic compound, namely, four equivalents of copper to one of tin, or 126'4 parts of cooper to 53..9 of tin, or 32 to 15 nearly. This is the alloy used by ewton in the first reflecting telescope. It is very brilliant, but so brittle as to break with a slight blow or sudden change of temperature, and so hard and friable that it cannot be worked with steel tools. The introduction of other metals or a larger proportion of copper may diminish tho brittleness, but will lower the brilliancy, and produce a metal more liable to tarnish. It is remarkable that speculum metal displays its porosity with the aid of a simple Coddington'a microscope. (BRONZE; SPECULUM METAL.] When it is stated that the 6 feet speculum of Lord Roase's telescope weighs 4 tons, it may be imagined that the casting of such a mass is a work of some difficulty. If a close mould were tieed, or if the metal were rapidly cooled, it would fly to pieces in attempting to work it, or if cast in nand, as an open casting, it would probably have a spongy or crystalline texture, which would bo visible when polished. Small specula are cast in lend, and as soon as they are set the sand core is pushed out of the central aperture of such as are intended for Grego rians, and the red-hot disc is surrounded by ignited wood ashes to delay the cooling; but io large specula the margins solidify first and from want. of ductility the central parts are torn away in the act of contracting, and the mass becomes rent or flawed. To prevent these defects and accidents, the speculum is cast on a chill or surface of iron moderately heated, the effect of which is to give a fine grain and increased compactness to the metal to a small distance from the surface. In Mr. Lassell's mode a cast-iron mould a little deeper than the speculum, with its bottom convex, and of the same radius, is attached to the end of a strong weighted lever, so that when the mould is empty, its bottom makes a considerable angle with the horizon, but becomes horizontal when charged with the proper quantity of speculum metal. The fused metal is poured into a lateral
cell, communicating with the mould at its lowest point. In this way the metal rises smoothly and evenly along the bottom, and entangled air and scoria are got rid of. In Lord Resse's arrangement the bottom of the mould is made of pieces of hoop iron on edge, wedged tightly in an iron frame, and turned to the proper curvature. This holds the fused metal, but allows the gaseous matter to pass freely through. On this a wooden pattern is laid about twice as deep as the intended speculum and 1.65th larger to allow for contraction : sand is rammed round this, and when the pattern is removed and the metal poured into the cavity, the lower surface is chilled, the sides in contact with the sand next harden, while the central parts remain longer fluid, and the top or back of the speculum congeals last, by which arrangement the contraction and irregularity of texture occur where they are least injurious. While the cast is still red-hot, in which state it is not brittle, it is transferred to an annealing furnace, which has been kept heated for some days so that its interior brickwork is at a full red, and here it is left to cool for a month or six weeks in case of a 6-feet speculum ; smaller ones, of course, requiring less time.
The grinding and the polishing, so as to produce a brilliant polish and a true paraboli6 figure, are still more difficult than the casting. The grinding is done by means of a tool on which are cemented small pieces of gritstone, dressed to the convexity of the surface by means of a gauge of sheet-iron. A convex and a concave gauge should be at hand : circular arcs are struck on them of a radius equal to twice the focal length, and afterwards left free by filing and grinding. When an even surface is produced by this tool, another is taken, of cast-iron, turned to the convex gauge, and cut up into small squares by two sets of grooves, about a quarter of an inch wide. This is charged first with sand, then with emery and water, and is made to traverse the face of the speculum by machinery. There must be occasional washing, and emery of increasing fineness used, till all scratches are removed. The face may be tested optically from time to time, and examined as to the focus. Early in the process the edge of the speculum is ground true with sand applied by means of a divided hoop of iron, which admits of being tightened. The back may also bo made uniform. In the grind ing, care was taken to prevent the slightest jar of the grinder on the :speculum, as the metal is so brittle that there is great danger of breaking it. A number of thin wooden wedges may be placed on the margin of the speculum, and the polisher slowly lowered upon them, and then by degrees they can be gently withdrawn.