In the polishing process, the tool, sometimes of lighter material than the grinder, is coated with pitch to a small thickness, but not closing the grooves which divide the surface into squares. The pitch must be of the degree of hardness such that a sovereign will stand on it on edge for one minute, and leave the impression of four nicks of the milling. The speculum is cleaned from emery or dust, and properly secured on the polishing-machine, and is smeared with a mixture of water and rouge (peroxide of iron), and the polisher, at the temperature of 80*, is placed on it for a short time. On raising it, all the squares of the pitch must have been in full contact with the speculum. If this be the case, the polisher is ready for work. In the old method of polishing by hand, the polisher is fixed to a firm block, and the ope rator, holding the speculum by a handle cemented to its back, works it backwards and forwards across the polisher by straight strokes, after a few of which he shifts his position to give them a new direction, turning the speculum somewhat to get rid as far as may be of inequalities in the abrasion ; and from time to time these cress-strokes are varied by circular ones; and as the moisture evaporates, a little water must be supplied at the edge. The adhesion and friction increase rapidly; the polishing-powder changes in colour, and at last disappears, when a fine polish covers the speculum, and the operator, by a few circular strokes, completes the parabolic figure.
This may be tested by optical means. Care must be taken not to pass the proper figure, for it cannot be recovered except by re-grinding and repeating the whole process. The advocates for this laborious kind of work attach much importance to the touch of the operator, since he can feel if anything is going wrong, and correct it in time. But as hand-polishing cannot be applied to specula above nine inches, and science required those of larger size, the introduction of machinery was necessary to progress. Tho two I ierschels availed themselves of mechanical aid; but the first astronomer who has given a minute account of his method is Lord Rosso, in the paper above referred to. His plan is to work by a double system of cross-strokes, while the speculum and polisher are slowly revolving with unequal velocities, The speculum is carried by a chuck attached to a strong vertical shaft. and is surrounded by water at 55° to insure the proper consistence of the pitch. The polisher is of iron, stiffened by means of ribs arranged like the walls of a honeycomb, and suspended by six points to lessen the chance of bending. The polishing-bar has a rectilinear motion from a variable crank. The effect of this crank corresponds to the cross-stroke in baud-polishing, tending to a spherical form; but the rotation of the speculum tends to lengthen the focus of the exterior zones of its surface. There is also an excentric motion, which aug ments to any required extent the circumferential action. The figure
depends on four things, namely, the radius of the primary, that of the excentric, and the angular velocities of it and the speculum. In polishing a six-feet speculum, the number of strokes is eight per minute; and for smaller ones, inversely as their diameter. The polishing of a six-feet speculum requires five hours. The figure of the speculum was tested in Lord Resse's process, during the grinding and polishing, by observing the reflection of a watch-dial placed perpendicularly over the speculum, at a height of about 90 feet. By carefully watching the image of this dial, the adjustment of the length of stroke could be made with such accuracy that the three-feet speculum, with its whole aperture, was thrown perceptibly out of focus by a motion of the eye piece amounting to less than the thirtieth of an inch. During the polishing, it was found necessary, in order to prevent irregular expan sion of the speculum, to maintain a uniform temperature in the polishing-room ; and it was also necessary to have a certain degree of moisture in the air, that the wet polishing-powder should dry at the proper rate. When the air was too damp, the polishing was not attempted ; if it were too dry, a jet of steam was introduced. Instead of water, ammonia soap (or common soap treated with ammonia) was sometimes used with the polishing-powder, as this was found to dry more rapidly.
Amore complete idea of the arrangements for grinding and polishing specula may be formed from Mr. Lassell's arrangement, as improved by 31r. Nasmyth, and represented in the figure (col. 704), the object being to imitate as closely as possible those evolutions of the hand by which Mr. Lassell had been accustomed to produce perfect surfaces on smaller specula. r L represent fast and loose pulleys for conveying the power, and transmitting it by the endless screw on the shaft, A, to the wheel c. The spindle, E, of this wheel has attached to it a crank or arm, r, which carries a pinion, J, and causes the pinion to revolve round the toothed circumference of the wheel which wheel II, being fixed to a bracket, causes the pinion J to revolve with as many turns as its circumference is leas than that of the wheel u, or five to one. The spindle of the pinion a has a wheel, s, fixed to it at its lower end, which wheel K, in like manner, conveys motion to the pinion L, which works on an adjustable centre-pin ; and as the T-groove in which the centre-pin of L works is radial to the centre of the wheel this pinion may be set to any degree of excentricity, and yet be in gear with IL The pinion L has also a cross-crank, st, attached to its under side, which, having its crank-pin, N, also sliding in a T-groove, it may bo set to and fixed at any degree of excentricity; so that by these two excentric movements we have the means of giving to the any compound motion required. The polisher is of wood or other suitable material, coated with pitch, and divided into squares.