Page:The American Cyclopædia (1879) Volume XV.djvu/268

 256 SPECULUM small specula had previously been successfully polished. The principles of the arrangement if 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 regu- lated. The tools for first smoothing the face of the speculum are made up of pieces of grit- stone, 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 how- ever carefully lifted, the figure would be almost certain to lose its accuracy by change of pres- sure 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 Herschel laid the speculum upon folds of woollen cloth, packing others closely all around it, filling the space between its edges and the box that con- tained it ; but this is not sufficient to preserve the form of the specula of 6 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 pe- culiarities in their forms and applications to use, according to the kind of reflecting tele- scope for which they are designed. It is evi- dent that as they reflect images as mirrors, the observer cannot be placed directly in front, and is not obvious how ho can take his position at the opposite end of the telescope, as in using those of the refractory kind. This is accom- plished in the reflecting telescope of Dr. James Gregory, known as the Gregorian telescope, by an aperture through the centre of the spec- ulum, 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 tele- scope, 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 spec- ulum. In Sir Isaac Newton's telescope the disk was also entire, and a small plain spec- ulum 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 ad- vantage 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. L6on 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 consider- ation before the British association at Dublin, who asserted from his own experience that the removal of a thickness of ^.f^g- or y^fo^ of an inch might seriously impair the accuracy of the defining power of the speculum. M. Fou- cault had preserved the silver mirrors for eight months without their being injured by tarnish- ing ; 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 perfec- tion the construction of silvered-glass reflec- tors ; 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. H. Draper of New York has con- structed large reflecting telescopes, and em- ployed 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 obser- vations requiring very great space-penetrating power. The subject of the speculum, in its