Mr. A. Ross discovered that the accumulation of the grinding powder near the middle of the glass, arose from the capillary attraction of the moistened powder, and that by the employment of the grinding powder in a dry state, the source of the most important error was removed. The grinding powder when used dry cuts less rapidly than when moistened, but from the greater exactness of the method, a much smaller amount of abrasion suffices to produce the plane surface, and consequently the dry process is but little more tedious than the wet.
In grinding and polishing the parallel disks of glass for sextants, the one surface is first ground flat, sometimes singly, but more generally from motives of economy five are ground at the same time. The disks are arranged in the order shown in the diagram, fig. 1110. The surface of the one tool having been wiped quite clean and dry, every disk is slightly moistened by breathing upon it. The disk is then placed upon the lower tool with moderate pressure; and if the disks be tolerably flat, the capillary attraction will suffice for retaining them in position during the grinding. A small quantity of finely washed emery is then dusted upon every disk, the second tool is placed over the whole, and attached to a line leading to a pulley placed overhead, and from which a counterpoise weight is suspended to regulate the pressure upon the disks, which should be only moderate. The upper tool is then rubbed with elliptical strokes continually varied in direction, and the tools are occasionally changed end for end, in order to place the surfaces in all possible relations to each other.
The surface upon which the glass disks are attached is always the lower tool; and the emery that is pushed off the disks falls on the lower surface, and is not picked up by the upper tool in the return stroke, which as previously mentioned, would be liable to round the extreme edges. By this arrangement it is only the tolerably uniform layer of emery that remains attached to the upper grinding tool that is employed, and the principal dependance for flatness is placed upon the condition of this tool. Water is used with the emery by most opticians, but by Mr. Ross the emery is used dry; and, to examine the progress of the work, the upper tool is removed, and the grinding powder blown away with a pair of bellows, as wiping with a cloth would leave particles of the powder attached to the face of the disks.
When the disks have been ground flat over their whole surfaces on one side, they are removed from the lower tool, which is thoroughly cleaned; the disks are then arranged as before, with their second faces upwards, and ground flat. But it will now happen that the two sides, although they may be plane surfaces, are not parallel to each other, and therefore the positions of the disks upon the lower tool are interchanged, 1 being placed in the position of 5, and 2 in that of 4, and the central disk is twisted round in the opposite direction. The whole are then ground in the same manner as before, until flat in their second positions, when the disks are again interchanged; and the process is repeated until both sides of the disks are made quite parallel to each other, and they collectively present a level surface in whatever order they may be arranged upon the lower tool. The disks are lastly polished either with oxide of iron, or putty powder.
Plane mirrors, made of speculum metal, and employed in reflecting telescopes, are ground to a plane surface upon flat grinding tools prepared as above described. The grinding tool is much larger than the specula to be ground, and is supplied with a small quantity of fine washed emery. The specula are rubbed singly upon the tool, with the fingers like a muller, until they are ground perfectly flat. The principal difficulty is, however, experienced in producing the high polish required in reflecting surfaces without impairing the accuracy of the plane surface obtained in grinding. The polisher is generally of cast iron, grooved over its entire surface, so as to divide it into squares, and covered with pitch, or a resinous cement, exactly in the same manner as in the polishers used for concave specula, the methods of working which will be briefly described in the fourth section of this chapter. Great importance is attached to the sizes of the polishers relatively to those of the specula, as if the polishers are made too large, the edges of the specula will be rounded off, or made convex; and if the polishers are made too small, the specula will be wrought concave.
Small plane specula are usually polished several at the same time. They are arranged close together to make up a circle, and with their faces quite level. The polisher is also circular; and Mr. A. Ross considers that the specula are the most accurately polished when the diameter of the polisher is about one-thirtieth greater than that of the circle of specula.
Elliptical specula, measuring above 3 inches by 2, are commonly polished singly, and Mr. Gambadella found that he succeeded the best when he employed a round polisher of the same size as the inner diameter of the oval. The Earl of Rosse found that he was enabled to polish specula of this size very perfectly with a polisher of three inches diameter applied in his machine for grinding and polishing concave specula briefly described in the fourth section of this chapter.
Speaking of polishing plane mirrors, the Earl of Rosse says: - "When the metal is polished, it is tested in the usual way by viewing an object alternately by direct and reflected vision, with a very good thirty inch achromatic, the aperture of which has been previously contracted to an inch and three quarters. If the metal is concave, it is worked with shorter strokes for about half an hour, and then tried; it will be found to have become less concave, possibly convex; in the latter case it is to be worked with longer strokes; thus, with the utmost facility, a metal can be worked alternately concave and convex; and, with a little practice, the limit between the two can be hit with such exactness, that, even with the severe test of a thirty inch achromatic, no deviation from the plane can be perceived, and the loss of light will be the only evidence that the rays have suffered reflexion before their incidence on the object glass." (Trans. Royal Society, 1840, p. 524.)