"By means of this admirable machine, 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. A stronger proof of the perfect capabilities of Mr. Lassell's machine could not be given."

Fig. 1136.

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From the foregoing description, it will be seen that the essen tial difference between the machines contrived by Lord Rosse and Mr. Lassell, is that in the former, the polisher is traversed over the speculum with reciprocating longitudinal motion, and in the latter, the polisher has a continuous epitrochoidal motion, the path of which is dependent upon the adjustments of L and M. Mr. Lassell's polisher was made of two thicknesses of pine wood, with the grain crossed; this, from its lightness, did not require to be counterpoised, and apparently from its being sufficiently yielding to accommodate itself somewhat to the form of the speculum, a single coating of pitch was found sufficient, and the polishing was completed with wet powder. Very complete evidence of the perfection of the speculum polished in this machine is afforded by the circumstance, that with the telescope to which it was fitted, Mr. Lassell discovered the satellite of Neptune, the eighth satellite of Saturn, and re-observed the satellites of Uranus, which latter, since their announcement by Sir W. Herschel had been seen by no other observer. These results have already arisen from the employment of Mr. Lassell's admirable contrivance and dexterity in the management of his polishing machine, and his excellent skill as an observer, in conjunction with a very perfect and powerful instrument, which has resulted principally from his own skilful exertions. The high value attached to these contributions to science, is evidenced by the circumstance that the Royal Astronomical Society awarded their gold medal for 1848 to Mr. Lassell.

Since the Earl of Rosse has shown that contrary to the previous general opinion, specula may be successfully polished by mechanical means, other machines have been constructed for the same purpose, but have not been applied to specula of such large dimensions. In Dr. R. Greene's machine for grinding and polishing specula and lenses, rewarded by the Society of Arts in 1834 (See Trans., Vol. L., p. 140), the polisher is mounted on a very slowly revolving axis, and the speculum also revolving slowly, but at a different rate, is traversed over the polisher by means of a central pin, joined to the extremities of two horizontal connecting rods at right angles to each other, actuated by two cranks, the relative velocities, length of stroke, and angular positions of which all admit of adjustment, and consequently the mirror can be traversed over the polisher in an infinite variety of curves,

A very simple machine for grinding and polishing specula of small size has been contrived by the Rev. William Hodgson, M.A., of Brathay, who has followed the general principles introduced by Lord Rosse, but has arranged the machine on the foundation of an ordinary turning lathe, driven by a foot-wheel, which, with the common overhead motion, and a part of the horizontal grinding machine shown in fig. 1039, page 1157, forms the principal portion of his polishing machine for specula. This contrivance, therefore, possesses the recommendation of being composed, in great measure, of the ordinary apparatus possessed by most amateurs, and may be readily fitted up for an occasional purpose, in those cases which would scarcely be considered of sufficient importance to call for the construction of the more elaborate machines of Lord Rosse or Mr. Lassell.

Fig. 1137 represents a modification of the arrangement of Mr. Hodgson's machine; the cast-iron frame, a, carrying the vertical mandrel of the horizontal grinding machine, is fixed at the back of the lathe-bearers, either by a bolt passing through the bearer; or a short supplementary bearer is fixed at the back, and the frame is held by a wedge beneath, as shown in fig. 1039. The speculum is mounted on a chuck, b, fixed on the screw of the vertical mandrel, in the usual manner, and the edge of the chuck is cut as a screw-wheel, which is driven by a tangent screw, mounted between the mandrel and popit-head of the lathe, and by which a slow rotatory motion is given to the speculum, c. The polisher, which is placed upon the speculum, is encircled by a loose ring, precisely similar to that employed in Lord Rosse's machine, and a reciprocating motion is given to the ring, which allows of the very slow rotation of the polisher, exactly as in Lord Rosse's arrangement.

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Fig. 1137.

The reciprocating motion of the polisher across the face of the speculum is obtained after the method adopted by Professor Willis for giving a reciprocating motion to his vertical sawing machine, shown in fig. 729, Vol. II., the only changes being those required in the alteration of the motion from the vertical to the horizontal position. For this purpose the spindle of the overhead motion is fitted with an adjustable eccentric, shown at d, a loop encircles the eccentric, and terminates in a catgut band that passes under the guide pulley e, and is connected to the front of the ring embracing the polisher. A second band proceeds from the back of the ring, and is connected to a vertical steel spring, f, fixed at the back of the lathe.

Motion is communicated to the lathe mandrel, by the band leading to the foot wheel in the ordinary manner, and a second band is led to the over-head motion, either from the foot wheel, as shown in the figure, or from the pulley of the mandrel. The relative velocities of the polisher and speculum, may be readily adjusted by shifting the bands to different grooves on the driving pulleys, and the length of stroke of the polisher is adjusted by shifting the position of the eccentric, which, as seen in the figure, is fixed on the front of a plain pulley by two clamping screws. The height of the guide pulley e, and the spring f, are of course required to be adjusted to the level of the polisher.