"For the second layer, it is mixed with one fourth of wheat flour, which by increasing its tenacity and diminishing its adhesiveness, prevents that accident so much complained of by practical men, viz., the separation of minute particles of pitch from the polisher, which afterwards run loose between the polisher and the speculum. It is to be boiled till the water of the flour has been expelled, and the mixture becomes clear, and the boiling further continued till some of the turpentine has been driven off, and the mixture has become so hard, that at a temperature of 55°, a very strong pressure of the nail makes but a slight impression: it is still too soft, and I then add to it an equal weight of resin; it will then be hard enough to produce a very true surface and at the same time, soft enough to suffer the particles of polishing powder to embed themselves, and consequently to produce a very fine black polish. Whenever the resinous mixture is remelted, I suspend the vessel to the beam of a scale, counterpoise it, and take care to apply the heat so gradually as not to drive off any of the turpentine, which is immediately perceptible by the disturbance of the equilibrium.'" "To apply the resin, the polisher is first heated to about 150° and the soft mixture laid on with a large flat brush, to about the thickness of about one-thirtieth, or one twenty-fifth of an inch; it is then suffered to cool to about 100°, and the hard mixture applied in the same way and to about the same thickness. When the temperature has sunk to 80°, the polisher is placed on the speculum previously covered with peroxide of iron and water, of about the consistence of thin cream." The Earl of Rosse found that the quality of the polish which yields the maximum of defining power is that technically called a black polish, provided a very fine grain is perceptible when the speculum is placed near a window. A speculum may be polished so that its surface appears without grain like quicksilver, but it is necessary for this purpose to employ a softer resinous cement than appears consistent with a very true surface, and the Earl of Rosse considers the best chance of improving the polish would be to search for some polishing substance consisting of smaller particles than the fine peroxide of iron, so as to produce a grain not exceeding the magnitude which theory has assigned as that of an undulation of light.

As shownin fig. 1132, the speculum revolves face upwards within the water cistern E, F, and this cistern being nearly filled with water kept at a temperature of 55° no unequal expansion of the speculum from increase of temperature can take place, and the pitch being also maintained at the same temperature as that at which it was first adjusted, does not become softened during the polishing as in the hand process.

In grinding and polishing the gigantic speculum of 6 feet diameter, the Earl of Rosse employed the same general arrangement of apparatus as that used for the 3 feet speculum; but from the increased dimensions, some modifications were required, the most important of which were noticed in a lecture on large reflecting telescopes, delivered by the Astronomer Royal before the Astronomical Society, and the substance of which was published in their Memoirs for March, 1849. From this, it appears that, in figuring the 6 feet speculum, the circular grooves in the cast-iron polisher were omitted, and the straight grooves at right angles were made about 1 inch deep, and 2 inches asunder, so as to divide the surface into squares. The weight of the polisher was uniformly supported at twelve points, the piece M, fig. 1132, being made triangular, with a pulley at each corner; a cord was passed over every pulley, and each end of the cord supported the middle of a straight lever, the ends of which were attached to the polisher.

In the rough grinding, the great weight of the iron disk, and the brittle nature of the speculum metal, rendered the placing of the grinder upon the mirror highly dangerous, as the slightest jar of the grinder upon the speculum would have been liable to break the latter. To avoid this risk, a number of thin wooden wedges were placed upon the margin of the speculum; the polisher was slowly lowered upon the wedges, and then, by degrees, they were gently withdrawn.

In the machine shown in fig. 1132, the bar, D G, passing through the fixed guide, D, at one end, and through the revolving guide, G, at the other, communicates a slow lateral motion to the grinder, alternately to the right and left. "But as, in the ordinary crank motion, the duration of the strokes at the extreme right and left would be too great, the wheel on the spindle of this grinding crank is elliptical, the proportion of its axes being about three to one; its angular motion is, therefore, unequal; and the strokes are thus made to dwell a shorter time near the extreme right and left, and a longer time near the center."

In polishing the speculum, it was found that not only should the temperature of the air in the polishing room be maintained nearly uniform during the process, in order to prevent the irregular expansion of the speculum, but also that it was essential that the degree of moisture in the air should be such, that the wet polishing powder should gradually dry at the proper rate. The polishing is therefore not attempted when the air in the room is too damp, and should the air be too dry, it is moistened by a jet of steam. After the process had been continued about eight hours, the polisher is removed, and a fresh application is made of the polishing powder mixed with "ammonia soap," a substance formed by treating common soap with ammonia. This dries more rapidly than the powder mixed with water alone, and the polishing is continued until the surface of the metal is very nearly dry, the 'process is then considered to be completed, and the polisher is taken off the speculum to allow of its inspection.