100 100 being stationary, both move with a regulated speed; the ring of the polisher, and therefore the polisher itself, has a transverse and a longitudinal motion; it makes eighty strokes in the minute, and twenty-four strokes and a-half backward and forward for every revolution of the mirror, and at the same time, 17% strokes in the transverse direction. The extent of the latter is 27 of the diameter of the Speculum. The polisher has another motion independent of the ring, for at the turn of the excentric, being for a little time free, it is carried for a short distance round, lying on the Speculum. In this way it makes one revolution for every fifteen of the mirror. The peculiarities of this mechanism are, 1st. the mechanism itself, as before this was used it was not thought possible to grind by machinery, -2nd. placing the Speculum with the face upward,-3rd. regulating the temperature by having it immersed in water, which is usually at 55° F. -4th. regulating the pressure and velocity. The pressure allowed to be exerted on a three-foot Speculum is ten pounds. The improvements effected in grinding and polishing mirrors by this machine are wonderfully great; and like all the other improvements invented by Lord Rosse, are the result of close calculation and observation. We are indebted to his powers of reasoning for all his works, and have to thank the blind goddess, Chance, for none. She is indeed so little enamoured of his Lordship that when she visits him at all, which is seldom, she appears in the shape of failure. The following quotation from Lord Rosse's publication in the Philosophical Transactions, will enable us to understand the theory of the Machine: - "Having observed that when the extent of the motions of the polishing machine was in certain proportions to the diameter of the Speculum, its focal length gradually and regularly increased, that fact suggested another mode of working an approximate parabolic figure. If we suppose a spherical surface, under the operation of grinding and polishing, gradually to change into one of longer radius, it is very evident that during that change, at no one instant of time will it be actually spherical and the abrasion of the metal will be more rapid at each point as it is more distant from the centre of the face. When, however, the focal length neither increases nor diminishes, the abrasion will become uniform over the whole surface producing a spherical figure. According, however, as the focal length (the actual average amount of abrasion during a given time being given) increases more or less rapidly, the nature of the curve will vary, and we might conceive it possible, having it in our power completely to control the rate at which the focal length increases, so to proportion the rate of that increase, as to produce a surface approximating to that of the paraboloid. Of course the chances against obtaining an exact paraboloid are infinitely great, as an infinite number of curves may pass between the parabola and its circle of curvature, and it is vain to look for a guide in searching for the proper one in calculations founded on the principles of exact science, as the effect of friction in polishing is not conformable to any known law; still from a number of experiments it might be possible to deduce an empirical formula practically valuable: this I have endeavoured to accomplish." The grinding of the Speculum to the proper figure then depends on the relative velocities of the different parts, as before stated. The substance made use of to wear down the surface was emery and water; a constant supply of these was kept between the grinder and the Speculum. The Grinder is made of cast iron, with grooves cut lengthways, across, and circularly, on its face; there are twentyfive grooves, crossed by as many more, which are quarter of an inch wide, and half an inch deep The circular grooves, of which there are thirteen concentric with the polisher, are three-eighths of an inch deep, and quarter of an inch wide. The polisher and speculum have a mutual action on each other; in a few hours, by the help of the emery and water, they are both ground truly circular, whatever might have been their previous defects. The grinding is continued until the required form of surface is produced, and this is ascertained in the following manner. There is a high tower over the house in which the Speculum is ground, on the top of this is fixed a pole, to which is attached the dial of a watch; there are trap doors which open, and by means of a temporary eye-piece at the calculated distance, allow the figure of the dial to be seen in the Speculum brought to a slight polish. If the dots on the dial are not sufficiently well defined the grinding is continued; but if it works satisfactorily, the polishing is commenced. This process, like the others, was in the beginning attended with much difficulty and annoyance. To polish Specula, the same tool which grinds them is covered with a layer of pitch, and on this is spread either putty or oxide of tin, as used by Newton, or oxide of iron, commonly called rouge, as used by Lord Rosse.Now, in trying to polish the three foot Speculum it was found that the pitch, which should be spread in a thin layer and very evenly over the surface, collected the abraded matter in some places more than in others, and of course, lost its required shape. If the pitch expanded laterally, so as to fit itself again to the Speculum, all would have gone on well; but, unfortunately, this could not take place unless the layer was made a great deal thicker than it could be satisfactorily used. In polishing small mirrors a thin layer could expand sufficiently laterally, on account of the small extent of its surface; but experience proved that the thickness of the layer should increase with the size of the Speculum, until, setting out from the depth of half-a-crown, which is requisite for a very small mirror, it would come to an |