The speculum is worked on the polisher with straight or elliptical strokes, the operator continually moving around the post to change the angle. Sufficient pressure must be uniformly applied, to keep the polisher fitted to the face of the speculum. After the rubbing has been continued some time, the polisher and speculum both become slightly warmed by the friction; and if the pitch was originally rather too hard to copy the figure of the speculum perfectly, the increased warmth will soften the. pitch, which will then ply well to the speculum, and the polishing will go on satisfactorily; but if the pitch becomes too soft, the figure of the speculum will be depreciated, and consequently great care is required to maintain the temperature of the polisher as uniform as possible, and just sufficient to keep the pitch in good working condition. Sometimes the polisher is very slightly warmed before applying it to the speculum.
The method of grinding and polishing specula by hand is at all times very difficult, and the results very uncertain, even with those of four or five inches diameter; as although it is comparatively easy to figure the specula so accurately to the general form that no errors can be detected by mechanical means, yet, when tried in the telescope, it frequently happens that so many minute errors are presented in the speculum, that the reflection appears quite undefined, and of course, in this condition, the speculum is unfit for its intended purpose.
The principal sources of error apparently inseparable from hand-polishing are, the absence of exact control in regulating the lengths and directions of the strokes, irregular increase of temperature in the speculum and polisher, unavoidably caused by the friction; and also the unequal pressure of the hand. All these difficulties rapidly increase with an enlargement of size; and a speculum of six or eight inches diameter is perhaps as large as can, with the utmost care, be produced by hand with the required accuracy. Larger specula have occasionally been polished by hand; but in the majority of instances it has ultimately proved that the increased incorrectness of defining power, has to a considerable extent counterbalanced the advantages derived from an increase of diameter.
With the view of avoiding the uncertainties of the hand-process, the Earl of Rosse constructed a machine for grinding and polishing specula, in which the different motions were susceptible of separate adjustments, and were all under complete control. A sketch of this machine was published in Sir D. Brewster's Journal for October, 1828. The machine was subsequently improved and enlarged, so as to be capable of working a speculum of three feet diameter; and from an experience of many years, during which specula were polished with it many hundred times with great accuracy, it was found perfectly successful in producing large specula with a degree of precision quite unattainable by hand, even by accident.
The machine is shown in fig. 1132, copied from his Lordship's paper on the reflecting telescope, published in the Philosophical Transactions of the Royal Society for 1840, from which the annexed description is also extracted: - "A is a shaft connected with a steam-engine; B, an eccentric, adjustable by a screw-bolt, to give any length of stroke from 0 to 18 inches; C, a joint; D, a guide; E F, a cistern for water, in which the speculum revolves; G, another eccentric, adjustable, like the first, to any length of stroke from 0 to 18 inches. The bar D G passes through a slit, and therefore the pin at G necessarily turns on its axis in the same time as the eccentric. H I is the speculum in its box, immersed in water to within one inch of its surface; and K L, the polisher, which is of cast iron, and weighs about two and a half hundred weight. M is a round disk of wood, connected with the polisher by strings hooked to it in six places, each two-thirds of the radius from the centre. At M there is a swivel and hook, to which a rope is attached connecting the whole with the lever N, so that the polisher presses upon the speculum with a force equal to the difference between its own weight and that of the counterpoise O. For a speculum three feet diameter I make the counterpoise ten pounds lighter than the polisher. The bar D G fits the polisher nicely, but without tightness, so that the polisher turns freely round, usually about once for every fifteen or twenty revolutions of the speculum, and it is prevented by four guards from accidentally touching the speculum, and from pressing upon the polisher by the two guides through which its extremities pass. In fig. 1131 this bar is on a larger scale. I have used a variety of contrivances for connecting the machinery with the polisher; but the one I have described is by far the best. The wheel B makes, when polishing a three-feet speculum, sixteen revolutions in a minute; to polish a smaller speculum, the velocity is increased by changing the pulley on the the shaft A. The machine is in a room at the bottom of a high tower, and doors can be opened in the successive floors, so that a dial-plate of a watch placed perpendicularly over the speculum can be examined at any moment. The dial-plate is attached to a mast, so as to be much higher than the tower, and about ninety feet from the speculum; and a small flat metal and eye-piece, with its proper adjustments, completes the arrangements for a Newtonian telescope."
The machine is driven by flat leather bands, as shown in the figure, an inspection of which will readily explain the action of the machine. The cast iron polisher is used first with emery and water for the grinding, and is afterwards coated with resinous cement for polishing, the intermediate process of the bed of hones not being required with the machine. No material difficulty was experienced in grinding the speculum to the spherical form; but some adjustments are required for obtaining the parabolical figure. The elliptical polisher of Mr. Edwards, although so valuable for figuring small specula by hand, was found to fail with large specula, from the radius of curvature being increased too rapidly near the edge. Speaking of this adjustment, the Earl of Rosse says: -