Chapter II: Glass-Grinding and Opticians' Work (3)

A bar of iron is then dovetailed loosely into the notch (Fig. 60, B), so that it will rest on the iron plate, and half fill the notch. The aperture thus left forms the port of ingress for the hot metal (see Fig. 61, M). A bit of sheet iron is attached to the upper surface of the ring, and lies as a sort of flap, shaped like a deep shovel, against the outside of the ring overhanging the port (Figs. 59 and 61 at F). This flap does not quite reach the iron plate, and its sides are bent so as to be in contact with the ring. A portion of a smaller ring is then applied in such a manner as to form a pouring lip or pool on the outside of the main ring at E, and the metal can only get into the main ring by passing under the edge of the flap and up through the port. This forms an efficient skimming arrangement. The process of casting is carried out by pouring steadily into the lip.

To avoid air bubbles it is convenient to cause the metal to spread slowly over the chill, and Mr. Nasmyth's method of accomplishing this is shown in the figure (61). The chill rests on three pins, A B C (Figs. 59 and 61). Before pouring begins the chill is tilted up off C by means of the counterpoise D, which is insufficient to tilt it after the speculum is poured. It is important that the chill should be horizontal at the close of the operation, in order that the speculum may be of even thickness throughout. This is noted by means of levels placed on the ring (at K for instance).

Fig. 61.

This apparatus may appear unnecessarily complex, but it is worth while to set it up, for it makes the operation of casting a speculum fairly certain. If the metal is at the right temperature it will form a uniformly liquid disc inside the ring. The mass sets almost directly, and as soon as this occurs it is pushed to the edge of the plate and the metal in the lip broken off by a smart upward tap with a hammer. The dovetailed bit of iron is knocked downwards and falls off, and the ring may then be lifted clear of the casting. The object of the dovetail will now be understood, for without it there is great risk of breaking into the speculum in knocking the "tail" off.

A box of quite dry sawdust is prepared in readiness for the process of annealing before the speculum is cast. The box must be a sound wooden or metal box, and must be approximately air-tight. For a speculum a foot in diameter the box must measure at least 3 feet both ways in plan, and be 2 feet 6 inches deep. Half the sawdust is in the box and is well pressed down so as to half fill it. The other half must be conveniently ready to hand. As soon as possible after casting, the speculum is thrown into the box, covered over with the sawdust, and the lid is put on.

The object in having the box nearly air-tight is to avoid air-currents, which would increase the rate of cooling. A speculum a foot in diameter may conveniently take about three days to anneal, and should be sensibly warm when the box is opened on the fourth day. For larger sizes longer times will be required. We will say that the sawdust thickness on each side must be proportional to the dimensions of the speculum, or may even increase faster with advantage if time is of no moment.

The process of annealing may be considered successful if the disc does not fly to pieces in working; it is to be worked on the chilled side. The object of giving the chill the approximate counterpart form will now appear; it saves some rough grinding, and causes the finished surface to be more homogeneous than it would be if the centre were sunk by grinding through the chilled surface.

In 1889 I learned from Mr. Schneider, Professor Row-land's assistant at Baltimore, that in casting specula for concave gratings a good deal of trouble had been saved by carrying out the operation in an atmosphere consisting mostly of coal gas. It was claimed that in this way the presence of specks of oxide was avoided. I did not see the process in operation, but the results attained are known and admired by all experimenters.

§ 71. Grinding and polishing Specula.

The rough grinding is accomplished by means of a lead tool and coarse emery; the size of grain may be such as will pass a sieve of 60 threads to the inch. The process of grinding is quite similar to that previously described, but it goes on comparatively quickly. The rough grinding is checked by the spherometer, and is interrupted when that instrument gives accordant and correct measurements all over the surface.

The fine grinding may be proceeded with by means of a glass-faced tool as before described, or the labour may be reduced in the following manner. A slate tool, which must be free from green spots (a source of uneven hardness), is prepared, and this is brought nearly to the curvature of the roughly ground speculum, by turning or otherwise. It is finished on the speculum itself with a little flour of emery. The fine grinding is then carried on by means of slate dust and water, the slate tool being the grinder. The tool is, of course, scored into squares on the surface.

If the casting process has been carried out successfully, the rough grinding may take, say six hours, and the fine grinding say thirty hours for a disc a foot in diameter. The greatest source of trouble is want of homogeneity in the casting, as evidenced by blowholes, etc. In general, the shortest way is to discard the disc and start afresh if there is any serious want of perfection in the continuity or homogeneity of the metal.

Fig. 62.

The finely ground surface must, of course, be apparently correct in so far as a spherometer (with 3 inches between the legs for a disc 1 foot in diameter) will show. Polishing and figuring are carried out simultaneously. Half an hour's polishing with a slate-backed pitch tool and rouge and water will enable an optical test to be made. The most convenient test is that of Foucault, a simple appliance for the purpose being shown in the figure (62). It essentially consists of a small lamp surrounded by an opaque chimney (A) through which a minute aperture (pin-hole) is made. A small lens may be used, of very great curvature, or even a transparent marble to throw an image of the flame on the pin-hole.

A screen (B) is placed close to the source, and is provided with a rocking or tilting motion (C) in its own plane. The source and screen are partly independent, and each is provided with a fine adjustment which serves to place it in position near the centre of curvature. The screen is so close to the pin-hole in fact that both the source and a point on the edge of the screen may be said to be at the centre of curvature of the mirror. The mirror is temporarily mounted so as to have its axis horizontal, in a cellar or other place of uniform temperature.

The final focussing to the centre of curvature is made by the fine adjustment screws; the image may be received on a bit of paper placed on the screen and overlapping the edge nearest the source. The screws are worked till the image has its smallest dimension and is bisected by the edge of the screen. The test consists in observing the appearance of the mirror surface while the screen is tilted to cut off the light, as seen by an eye placed at the edge of the screen, a peephole or eye lens being provided to facilitate placing the eye in a correct position. The screen screws are worked so as to gradually cut off the light, and the observer notes the appearance of the mirror surface. If the curves are perfect and spherical, the transition from complete illumination to darkness will be abrupt, and no part of the mirror will remain illuminated after the rest.

For astronomical purposes a parabolic mirror is required. In this case the disc may be partially screened by zonal screens, and the position of the image for different zones noted; the correctness or otherwise of the curvature may then be ascertained by calculation. A shorter way is to place the source just outside the focus, to be found by trial, and then, moving the extinction screen (now a separate appliance) to, say, five times the radius of curvature away, where the image should now appear, the suddenness of extinction may be investigated. This, of course, involves a corresponding modification of the apparatus.

Whether the tests indicate that a deepening of the Centre, i.e. increase of the curvature, or a flattening of the edges is required, at least two remedial processes are available. The "chisel and mallet" method of altering the size of the pitch, squares of the polisher may be employed, or paper or small pitch tools may be used to deepen the centre. The "chisel and mallet" method merely consists in removing pitch squares from a uniformly divided tool surface by means of the instruments mentioned. This removal is effected at those points at which the abrasion requires to be reduced.

When some practice is attained, I understand that it is usual to try for a parabolic form at once, as soon as the polishing commences. This is done by dividing the pitch surface by V-shaped grooves, the sides of the grooves being radii of the circular surface, so that the central parts of the mirror get most of the polishing action. If paper tools are used they must not be allowed much overhang, or the edges of the mirror betray the effects of paper elasticity. Most operators "sink" the middle, but the late Mr. Lassell, a most accomplished worker, always attained the parabolic form by reducing the curvature of the edges of a spherical mirror.

§ 72. Preparation of Flat Surfaces.

As Sir H. Grubb has pointed out, this operation only differs from those previously described in that an additional condition has to be satisfied. This condition refers to the mean curvature, which must be exact (in the case of flats it is of course zero) to a degree which is quite unnecessary in the manufacture of mirrors or lenses.

A little consideration will show that to get a surface flat the most straightforward method is to carry out the necessary and sufficient condition for three surfaces to fit each other impartially. If they each fit each other, they must clearly all be flat. To carry out the process of producing a flat surface, therefore, two tools are made, and the glass or speculum is ground first on one and then on the other, the tools being kept "in fit" by occasional mutual grinding. The grinding and polishing go on as usual. If paper is employed, care must be taken that the polisher is about the same size as the object to be polished.

There is a slight tendency to polish most at the edges; but if the sweeps are of the right shape and size, this may be corrected approximately. The best surfaces which have come under my notice are those prepared as "test surfaces" by Mr. Brashear of Alleghany, Pa, U.S.A. These I believe to be pitch polished. A pitch bed is prepared, I presume, in a manner similar to that described for rocksalt surfaces; but the working of the glass is an immense art, and one which I believe--if one may judge by results--is only known to Mr. Brashear.

In general, the effect of polishing will be to produce a convex or concave surface, quite good enough for most purposes, but distinctly faulty when tested by the interference fringes produced with the aid of the test plate. The following information therefore--which I draw from Mr. 'Cook--will not enable a student to emulate Mr. Brashear, but will undoubtedly help him to get a very much better surface than he usually buys at a high price, as exhibited on a spectroscope prism.

The only difference between this process and the one described for polishing lenses, lies in the fact that the rouge is put into the paper surface while the latter is wet with a dilute gum "mucilage." It is of course assumed that the object and the two tools have been finely ground and fit each other impartially. The paper is rubbed over with rouge and weak gum water. The tool, when dry, is applied to the flat ground surface (of the object), and is scraped with the three-cornered file chisel as formerly described. This process must be very carefully carried out. The paper must be of the quality mentioned, or may even be thinner and harder. The cross strokes should be more employed than in the case of the curved surfaces.

A good deal will depend on the method employed for supporting the work; it is in general better to support the tool, which may have a slate backing of any desired thickness, whereby the difficulty resulting from strains is reduced. The work must be mounted in such a way as to minimise the effect of changes of temperature. If a pitch bed is selected, Mr. Brashear's instructions for rock salt may be followed, with, of course, the obvious necessary modifications. See also next section.

§ 73. Polishing Flat Surfaces on Glass or on Speculum Metal.

The above process may be employed for speculum metal, or pitch may be used. In the latter case a fresh tool must be prepared every hour or so, because the metal begins to strip and leave bits on the polisher; this causes a certain amount of scratching to take place. As against this disadvantage, the process of polishing, in so far as the state of the surface is concerned, need not take an hour if the fine grinding has been well done.

For the finest work changes of temperature, as in the case of glass, cause a good deal of trouble, and the operator must try to arrange his method of holding the object so as to give rise to the least possible communication of heat from the hand.

The partial elasticity of paper, which is its defect as a polishing backing, is, I believe, partly counterbalanced by the difficulty of forming with pitch an exact counterpart tool without introducing a serious rise of temperature (i.e. warming the pitch). The rate of subsidence of the latter is very slow at temperatures where it is hard enough to work reliably as a polisher.

A student interested in the matter of flat surfaces will do well to read an account of Lord Rayleigh's work on the subject, Nature, vol. xlviii, 1893, pp. 212, 526 (or B. A. Reports, 1893). In the first of these communications Lord Rayleigh describes the method of using test plates, and shows how to obtain the interference fringes in the clearest manner.

For the ordinary optician a dark room and a soda flame afford all requisite information; and if a person succeeds in making three glass discs, say 6 inches in diameter, so flat that, when superposed in any manner, the interference fringes are parallel and equidistant, even to the roughest observation, he has nothing to learn from any book ever written on glass polishing. Lord Rayleigh has also shown how to use the free clean surface of water as a natural test plate.

Since the above was written the following details of his exact course of procedure have been sent to me by Mr. Brashear, and I hereby tender my thanks:-

"It really takes years to know just what to do when you reach that point where another touch either gives you the most perfect results attainable, or ruins the work you have already done. It has taken us a long time to find out how to make a flat surface, and when we were called upon to make the twenty-eight plane and parallel surfaces for the investigation of the value of the metre of the international standard, every one of which required an accuracy of one-twentieth of a wave length, we had a difficult task to perform. However, it was found that every surface had the desired accuracy, and some of them went far beyond it.

It is an advantage in making flat surfaces to make more than one at a time; it is better to make at least three, and in fact we always grind and 'fine' three together. In making speculum plates we get up ten or twelve at once on the lead lap. These speculum plates we can test as we go on by means of our test plane until we get them nearly flat. In polishing them we first make quite a hard polisher, forming it on a large test plane that is very nearly correct. We then polish a while on one surface and test it, then on a second and test it, and after a while we accumulate plates that are slightly concave and slightly convex. By working upon these alternately with the same polisher, we finally get our polisher into such shape that it approximates more and more to a flat surface, and with extreme care and slow procedure we finally attain the results desired.

All our flats are polished on a machine which has but little virtue in itself, unmixed with brains. Any machine giving a straight diametrical stroke will answer the purpose. The glass should be mounted so as to be perfectly free to move in every direction--that is to say, perfectly unconstrained. We mount all our flats on a piece of body Brussels carpet, so that every individual part of the woof acts as a yielding spring. The flats are held in place by wooden clamps at the edges, which never touch, but allow the bits of glass or metal to move slowly around if they are circular; if they are rectangular we allow them to tumble about as they please within the frame holding them.

For making speculum metal plates either plane or concave we use polishers so hard that they scratch the metal all over the surface with fine microscopic scratches. We always work for figure, and when we get a hard polisher that is in proper shape, we can do ever so many surfaces with it if the environments of temperature are all right. If we have fifty speculum flats to make, and we recently made three times that number, we get them all ready and of accurate surface with the hard polisher. Then we prepare a very soft polisher, easily indented when cold with the thumb nail. A drop of rouge and about three drops of water are put on the plate, and with the soft polisher about one minute suffices to clean up all the scratches and leave a beautiful black polish on the metal. This final touch is given by hand; if we do not get the polish in a few minutes the surface is generally ruined for shape, and we have to resort to the hard polisher again.

I assure you that nothing but patience and perseverance will master the difficulty that one has to encounter, but with these two elements 'you are bound to get there.'"