Speculum (Lat., mirror), a term commonly applied to concave metallic reflectors, such as are used in reflecting telescopes for concentrating the rays of light from distant luminous bodies, and presenting the image of these in their focus. Their perfection consists in large surface, whereby they collect the greatest quantity of light; in the highest possible polish, whereby it is reflected with least loss; and in the most exact parabolic curvature, rendering the image distinct and precise. In a speculum of 6 ft. diameter, a variation even at its edge from the true parabolic curvature, so minute as to escape detection with any except the most refined means of measurement, may render the whole useless. The metallic alloy best adapted for the requirements of specula was first employed for this purpose by Sir Isaac Newton, and is similar to that used by the ancient Egyptians for mirrors. It consists of copper and tin, to which Newton added a little arsenic, and sometimes silver or zinc; but Lord Rosse found that the two metals first named are better without the addition of any other. The volatile metals arc objectionable. Silver makes the alloy too soft; nickel, though it whitens the yellow alloy of copper, makes the speculum alloy yellowish.
Rosse was particular that the copper and tin should be combined in their atomic proportions (4 atoms of copper =126.8, to 1 of tin =59), and the purest metals should be selected; for the smaller specula it is even recommended that the copper be obtained by the electrotype process; this is hardly practicable for the large ones. The alloy is remarkable for its extreme brittleness and hardness. Large masses of it sometimes break from a slight blow or sudden change of temperature; and it is so hard that it cannot be wrought with tools of steel. It takes a most brilliant polish, which it has been known to retain with little tarnish, though exposed to the air for more than 16 years. A large speculum, however, ought always to be covered when not in use, and the air about it should be kept dry by means of an open box of quicklime. Great difficulties have been encountered in preventing the large specula from changing their form by their own weight, and those of 6 ft. diameter are made so thick, to give them the necessary stiffness (though supported when finished by the most ingenious appliances), that they are among the heaviest of bronze castings; and the preparation of the rough mass is among the most difficult of foundery operations.
Rosse's six-foot speculum weighs 4 tons; one of 3 ft. 3¾ in. thick, weighs 13 cwt.; and one of 2 ft., 3¼ cwt. The alloy is prepared by melting the metals separately, and pouring the tin into the copper, stirring rapidly, and then, before the tin oxidizes, casting the alloy into ingots. It is tested when cold to ascertain its brilliancy, and more tin is added if necessary. The best mode of preparing the moulds has been arrived at from long experimental trials in the casting of the smaller specula. It was found that sand moulding would not answer for the surface of the disks, as the texture of the alloy near the outside was rendered somewhat spongy and crystalline; and though this was so slight as to be detected only by the microscope, it still seriously impaired the polished surface. The face of the disk at least must then be " chilled," as cast iron is chilled, by pouring it into metallic moulds to increase the density of its surface. But the ordinary temperature of the atmosphere was found to be too low for the moulds to receive this alloy, and they were consequently heated to about 212° to prevent too sudden cooling and consequent irregular contraction. For moderate-sized specula cast-iron moulds were used, necessarily open, or the casting would inevitably fly in pieces.
They were made a little deeper than the speculum, with the bottom of the same convexity with this, and so supported that they could be instantly filled from the lowest point, and turned into a horizontal position when charged with the proper weight of the metal. The air and any foreign substance present are thus carried up to the surface, and separated from the alloy. But this was not sufficiently perfect for the largest castings, and Lord Rosse adopted for these the following method. An iron frame of sufficient diameter was filled with pieces of hoop iron set on edge and tightly wedged together, and the upper surface was turned off to the curvature of the face of the speculum. This was to serve for the bottom of the mould, being tight enough to hold the melted metal, while it allowed the escape of the gases through the interstices uniformly over the whole face. Upon this bottom was laid the wooden pattern, made twice as deep as the intended speculum, and with an allowance of 1/65 in the diameter for shrinkage. The sides of the mould were then formed by ramming sand around the pattern.
By this arrangement the first cooling is on the under face, next on the sides, and the final congealing is on the top or back, where the contraction and resulting irregularities will be concentrated in the least important part. The metal while red-hot is removed to a furnace specially prepared for it, the bottom having the curvature of the disk (unless in case of using an iron mould, when this too is taken along). By fires already kept up several days, the inner walls of the furnace should be at a full red heat. The vacant spaces around the casting are then filled with ignited fuel, and every aperture is carefully luted. A large speculum should thus be left to cool for a month to six weeks; and the result may still be unsatisfactory if the walls of the furnace are less than 2 ft. thick. - The production of the true parabolic figure, combined with a brilliant polish, is attained by grinding succeeded by polishing. Machines applicable to this object have been invented by Sir W. and Sir J. Herschel, Lord Rosse, Mr. Lassell, an amateur optician and astronomer, Mr. De la Rue, Mr. Grubb of Dublin, and others, which are too complicated to be particularly described in this place.
The object sought for is to restrict the operation of the rubbing tools to the production of the particular curvature required, and insure a uniform action upon every part of the surface of the disk. The speculum, placed upon a slowly revolving platform, presents its face to the action of the rubber above it, which by Lord Rosse's arrangement was caused to vibrate regularly in one and the other direction, while at the same time it revolved at a different rate from that of the speculum. Mr. Lassell caused the rubber to revolve in small circles, while the speculum, turning on its axis, which was not in line with that of the rubber, presented successively all portions of its surface to this circling action of the rubber, thus imitating the movements of the hands by which the small specula had previously been successfully polished. The principles of the arrangement of Mr. Lassell were so mathematically exact, that, as stated by Mr. Nasmyth, "a speculum having a decidedly hyperbolic figure may be corrected and brought to a perfect parabola, or to a spherical curve, or the same may be done in the reverse order at pleasure." The test of the polishing consists in observing through an eye piece the reflection of the dial of a watch set directly over the speculum, in the case of Lord Rosse's operations at the height of 90 ft.
The success of the polishing was dependent on the state of the atmosphere as regards temperature and moisture, both of which required at times to be artificially regulated. The tools for first smoothing the face of the speculum are made up of pieces of gritstone, cemented together in a frame and dressed on the face to the proper degree of convexity. The next are disks of cast iron, their face also of the exact curvature, and grooved by two lines of furrows a quarter of an inch wide and the same deep, crossing each other at right angles. These are fed with sharp quartz sand, and afterward with emery and water. When the work has proceeded to the use of very fine emery the scratches disappear, and the rubber is in perfect and uniform contact with the speculum. For polishing, the, cast-iron rubber was used by Lord Rosse, counterpoised and provided with circular grooves in addition to the rectilinear ones. Its face was coated with a thin layer of pitch, with another upon this of rosin and flour, which serves as the bed for the polishing powder or rouge to imbed itself. Mr. Lassell's polisher was of pine wood in two layers, the grain crossing, and the face coated with pitch above.
The preparation of these polishers involves nice operations, which may not be neglected without great risk of failure. The largest specula when polished ought never to be removed from their supports; for however carefully lifted, the figure would be almost certain to lose its accuracy by change of pressure in the mass. Even one of 9 in. diameter, when supported by the pressure of springs against three stops bearing on its edges, loses its defining power. Sir John Ilerschel laid the speculum upon folds of woollen cloth, packing others closely all around it, filling the space between its edges and the box that contained it; but this is not sufficient to preserve the form of the specula of G ft. diameter and as many inches thick, and the contrivances for this are a most complicated system of bearings, springs, and levers. - Specula exhibit some peculiarities in their forms and applications to use, according to the kind of reflecting telescope for which they are designed. It is evident that as they reflect images as mirrors, the observer cannot be placed directly in front, and it is not obvious how he can take his position at the opposite end of the telescope, as in using those of the refractory kind.
This is accomplished in the reflecting telescope of Dr. James Gregory, known as the Gregorian telescope, by an aperture through the centre of the speculum, and the introduction of a small concave speculum in the centre of the great tube, facing the large speculum, and a little in advance of its focus. Back of the great speculum the tube is extended of reduced diameter, and in its extremity is a magnifying eye piece, by which the image reflected from the small speculum through the aperture of the large one reaches the eye. In Sir William Herschel's great telescope, with' its 4-ft. speculum and 40-ft. focal length, the disk was entire, and the image was reflected direct to an eye piece at the mouth of the tube and near one side of it, so as not to intercept too much light. This was effected by a slight inclination of the plane of the speculum. In Sir Isaac Newton's telescope the disk was also entire, and a small plain speculum reflected the cone of rays sent from it, before meeting in the focus, to the eye piece placed in the upper side of the tube.
Casse-grain's telescope differs from Gregory's in the small reflector being convex instead of concave. - Specula have recently been made of polished silver surface, which has the advantage over that of the speculum metal of reflecting 91 per cent, of the incident light, instead of 67 per cent. The silver, after the method of M. Leon Foucault, is laid in a very thin uniform coating upon a speculum of glass, figured and polished to a true parabola. This is done by Drayton's process of precipitating the metal from the solution in nitric acid by oil of cassia. The precipitated silver is polished by gentle rubbing with a skin lightly tinged with oxide of iron, and soon acquires a very brilliant lustre without material change of figure. This, however, was questioned by Mr. Grubb, when the subject was under consideration before the British association at Dublin, who asserted from his own experience that the removal of a thickness of 1/40,000 or 1/50,000 of an inch might seriously impair the accuracy of the defining power of the speculum. M. Foucault had preserved the silver mirrors for eight months without their being injured by tarnishing; but whenever this might occur they were easily polished again, and the silver itself could be at any time renewed.
Mr. Browning of London has carried to a high degree of perfection the construction of silvered-glass reflectors; and the experience of those acquainted with instruments constructed on his plan is strongly in favor of the method. In America the reflecting telescope has not hitherto met with much favor; though the success with which Dr. II. Draper of New York has constructed large reflecting telescopes, and employed them even in the delicate work of lunar photography, promises before long to enlist American ingenuity in the improvement of a class of telescopes which must probably always have the preference over refractors for observations requiring very great space-penetrating power. - The subject of the speculum, in its mathematical and mechanical details, is treated in various memoirs in the English scientific journals, from the time of Newton's paper in the "Philosophical Transactions" of 1G72 to the present day. Lord Posse's papers are contained in the "Edinburgh Journal," vol. ix., 1828, and vol. ii. (new series), 1829, and in the "Philosophical Transactions," 1840 and 1850. The mechanical details are fully described in Holtzapffel's MechanicalManipulations." For Lassell's process, see " Transactions of the Royal Astronomical Society," 1849.