In the figuration of small specula by hand, the metal is usually attached by cement to a temporary back, which serves as the support during the grinding and polishing, and is removed before the speculum is placed in the telescope, but even with small specula there is always some risk of distorting or breaking the speculum in the act of detaching the back, and it is therefore at all times the better practice, to form the back in such a manner, that it may remain permanently attached to the speculum, and constitute its bed in the telescope. Larger specula, unless uniformly supported at all times, are liable to flexure, which would destroy the accuracy of figure given by grinding; it is therefore of the first importance that the larger specula should be ground and polished in the same bed that is to be employed in the telescope.
To prevent flexure in specula of moderate dimensions, the Earl of Rosse found it quite sufficient to support them in their box, on three strong iron plates, each plate being one-third part of a circular area, the same size as the speculum, and a sector of it; the plates rest at their centers of gravity, on points fixed at the bottom of the box of the speculum, and therefore no flexure of the box can affect the speculum. In supporting the speculum of 3 feet diameter, Lord Rosse attached nine plates to the speculum, every group of three being supported at their centers of gravity upon a triangle, having three points to sustain the pressure, and the center of every triangle is supported upon one of three points in the bottom of the box. The 6 foot speculum is supported in a similar manner, upon twenty-seven cast-iron plates, sustained upon a series of nine triangles, that are again supported upon three triangles, the centers of which rest upon three points. The twenty-seven plates were originally attached to the speculum by felt and pitch, but when the telescope was placed at different inclinations, it was found that the reflection was distorted, owing to the speculum having a slight motion edgeways, which threw some of the points of bearing partially out of contact. This difficulty has been overcome by removing the layer of pitch and felt, by which the plates were attached to the speculum, and substituting sheets of tin, which allow the speculum to slide a small distance upon the plates.
1289 MR. lassell's machine for polishing specula.
A very valuable machine, of a different construction, for polishing specula, has been contrived by Mr. William Lassell, of Starfield, near Liverpool. The attention of this gentleman has been for many years devoted to the construction of reflecting telescopes, and his success in figuring by hand specula of all sizes, up to 9 inch diameter and 9 feet focal length, led him to conceive the idea of constructing a telescope with a speculum of 2 feet diameter, and 20 feet focal length.
As a preliminary step to the construction of the speculum, Mr. Lassell. inspected Lord Rosse's laboratory, and the performance of the machinery for grinding and polishing specula appeared so satisfactory, that Mr. Lassell determined to employ a similar machine for polishing his 2 foot speculum. "But finding, after many months' trial, that he could not succeed in obtaining a satisfactory figure, he was led to contrive a machine for imitating as closely as possible those evolutions of the hand by which he had been accustomed to produce perfect surfaces on smaller specula." The idea of the machine was communicated by Mr. Lassell to his friend Mr. James Nasmyth, of Patricroft, near Manchester, by whom the mechanical details were designed, and the machine constructed on the beautiful arrangement shown in fig. 1135, which is copied from a drawing kindly supplied by Mr. Nasmyth, and we are also indebted to the same gentleman for the annexed description, which he has obligingly written for these pages.
"The power is conveyed, in the first instance, by a band or belt, to the pulley A, which conveys motion by the endless screw B, to the wheel C. The spindle of the wheel C, viz., D, has made fast to it, a crank, or arm, E, which carries a pinion F, and causes the pinion to revolve round the toothed circumference of the wheel G, which wheel G being fixed to the bracket H, causes the pinion F to revolve with as many turns as its circumference is less than that of the wheel G, viz., 5 to 1.
"As the spindle of the pinion F, has a wheel K, fixed to it a its lower end, this wheel K will, in like manner, convey motion to the pinion L, which works on an adjustable center pin, and as the T groove in which the center pin of L works, is radial to the center of the wheel K, this pinion may be set to any degree of eccentricity, and yet be in gear with K.
1291 MR. Lassell's machine for polishing specula.
"It will also be seen that the pinion L has a cross crank, M, attached to its under side, which, having its crank pin, N, also sliding in a T groove, it may be set to, and fixed at, any degree of eccentricity, so that we have by these two eccentric movements the means of giving to the pin N, any compound motion we require.
"The polisher is of wood, or other suitable material coated with pitch, and divided into squares. This polisher is free to move upon the pin N, while N causes the polisher to slide over the surface of the speculum with a motion somewhat like that shown in fig. 1136.
"In order to cause every part of the surface of the speculum to continually change its situation with respect to the movements of the polisher, it has also a slow revolving motion given by an endless screw, P, pitched or working into the teeth of the wheel R, which forms the base on which the speculum rests, while receiving the action of the polisher.
"The speculum rests on nine equilibrium points so that each ninth of its body is made to rest on a point or surface placed under the center of gravity of each ninth of the speculum surface, and so avoid all risk of distortion. It is the best practice to polish the speculum while resting in the cell in which it is to be when actually in the telescope, so as no risk of distortion may occur, as would be the case were it removed, after polishing, into another cell or bed.