This polisher is free to move upon the pin N, while N causes the polisher to slide over the surface of the speculum a with a motion not like that represented in the figure contained in Holtzapfrel's Mechani all Manipulation,' where the curves are all re-entering, but rather as in the following figure, where the curves do not re-enter. In order that every part of the surface of the speculum may continually change its position, with respect to the movements of the polisher, a slow revolving motion is given to it by an endless screw 011 B working into the teeth of the wheel n, which forms the base on which the speculum rests. It I rests, in fact, on nine equilibrium points in the cell in which it is to rest when actually in the telescope, so as to incur no risk of distortion. " By means of this arrangement,' says 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." In Reese's arrangement, the polisher is travetsed over the speculum with reciprocating longitudinal motion; and in Mr. Lassen's, the polisher has a continuous epitroehoidal motion, the path of which is dependent upon the adjustments of L and lc The polisher is made of two thicknesses of pine-wood, with the grain crossed. This, from its lightness, does not require to be counterpoised, as in Reese's ; and, apparently from its being sufficiently yielding to accom modate itself somewhat to the form of the speculum, a single coating of pitch was found sufficient, and the polishing was completed with wet powder. The value of the two-feet speculum thus produced is
shown by Mr. Lassell's discovery of the satellite of Neptune, an eighth satellite of Saturn, and the re-observation of the satellites of Uranus, which had not been seen since their announcement by Sir W. Herschel. Mr. Lassell's polisher has served as a type to other skilful mechanicians, such as Mr. De la Rue;Mr. Grubb, of Dublin, and Mr. Lassell himself has introduced some modifications.
Further details respecting specula belong to TELESCOPE; but we may mention the proposal, if not introduction, of silver as a substitute for speculum metal. It reflects of the incident light, while speculum metal only reflects 0.67. It is, on the other hand, liable to tarnish, and there are difficulties in the way of figuring and polishing it. N. Foucault, however, has formed a speculum of glass figured and polished to a true parabola, and deposited a thin but uniform film of silver on its surface, by means of Drayton's process, in which a solution of nitrate of silver is reduced by means of oil of cassia. Tho exterior surface of the glass speculum is parabolic, but has no lustre. That property may be imparted to it in a high degree without any sensible change of figure, by means of light friction with wash-leather and a little peroxide of iron. In case of tarnish, the polish may be renewed by the same process, and often be repeated before the silvering requires to be renewed.