Consequently we now enter on the stage of fine grinding by hand. A leaden pedestal, for the sake of stability, must be provided on which to mount the lens, so that the surface to be operated on may be nearly horizontal (Fig. 50). Before this can be done, however, fresh grinding tools (two for each surface) must be properly prepared.

After trying several plans I unhesitatingly recommend that all fine-grinding surfaces should be made of gla.s.s. This is easily done by taking two discs of lead, or iron, or slate, cut to a one-tenth inch smaller radius of curvature (in the case of a convex tool, and the opposite in the other case) than the lens surface (Fig. 51, A).

On these, square bits of sheet gla.s.s, one-tenth of an inch thick, are to be cemented, so as to leave channels of about one-eighth of an inch between each bit of gla.s.s (Fig. 52, B). The "mastic" cement formerly described may be employed for this purpose.

Fig. 50.

The bits of gla.s.s ought first to have their edges dressed smooth on the grind-stone. A convex and concave gla.s.s surface having been thus roughly prepared, they must be mounted in turn in the lathe, and brought to the proper curvature by grinding with the tools formerly employed and tested by the template or spherometer. It is well to control this process by means of a spherometer, so that the desired radius may be approximately reached. The two gla.s.s-grinding tools are then ground together by hand (see -- 53 and -- 61), the spherometer being employed from time to time to check the progress of the work.

In general, if large circular sweeps are taken, greatly overhanging the side of the gla.s.s surface to be figured, both the upper and lower surfaces will be more ground at the edges, while in the opposite event the centre will be chiefly affected.

Fig. 51.

A spherometer capable of measuring a 2-inch surface may be procured, having a screw of, say, 50 threads to the inch, and a micrometer surface divided into 200 parts, each part easily capable of subdivision--into tenths or even twentieths. To get the full advantage of the spherometer it must screw exceedingly freely (i.e.

must be well oiled with clock oil), and must not be fingered except at the milled head. If one of the legs is held by the fingers the expansion is sufficient to throw the instrument quite out of adjustment. The gla.s.s-grinding tools being brought to the proper figure, the next process is to transfer the same to the lens, and this is done by similar means, the fellow tool being used to correct the one employed in grinding the lens surface. Before the grade of emery is changed all three surfaces must agree, as nearly, at least, as the spherometer will show.

In order to prevent confusion the following summary of the steps already taken may be given. The discs of gla.s.s are first ground or turned so as to be truly circular. Four "tools" are made for each surface--a rough pair of iron or lead, and a finishing pair of iron, lead, or slate faced by gla.s.s squares. For a small lens the iron or lead backing may be used, for a large one the slate. The rough tools are used to give an approximate figure both to the lens and to the finishing tools.

The final adjustment is attained by grinding one of the gla.s.s-faced tools alternately upon the lens and upon the fellow gla.s.s-faced tool.

The spherometer is accepted at all stages of the process as the final arbiter as to curvature. Some hints on the form of strokes used in grinding will be given later on (see -- 61). It suffices to state here that the object throughout is to secure uniformity by allowing both the work and the tool to rotate, and exercising no pressure by the fingers. The tool backing may weigh from one to two pounds for a 2-inch lens.

-- 60. The tools and lens being all of the same curvature, the state of the surface is gradually improved by grinding with finer and finer emery. The best way of grading the emery is by washing it with clean water, and allowing the emery (at first stirred up with the water) to settle out. The longer the time required for this part of the process the finer will be the emery deposited. An ordinary bedroom jug is a very good utensil to employ during this process; a large gla.s.s jug is even better. The following grades will be found sufficient, though I daresay every operative's practice differs a little on this point.

1st grade: Flour emery, with the grit washed out, i.e. allowed to stand for 2" (sec.) before being poured off.

2nd grade: Stand 5" (secs.), settle in 1' (min.)

3rd grade: Stand 1', settle in 10'.

4th grade: Stand 10', settle in 60'.

It is generally advisable to repeat the washing process with each grade. Thus, selecting grade 2 for ill.u.s.tration, the liquor for grade 3 must be poured off without allowing any of the sediment to pa.s.s over with it. If any sediment at all pa.s.ses, one has no security against its containing perhaps the largest particle in the jug. As soon as the liquor for No. 3 has been decanted, jug No. 2 is filled up again with clean water (filtered if necessary), and after standing 5" is decanted into jug No. 2b, the sediment is returned to jug No. 1, and the liquor, after standing 1', is transferred to jug No. 3.

The greatest care is necessary at each step of the operation to prevent "sediment" pa.s.sing over with liquor. There is a little danger from the tendency which even comparatively large particles of emery have to float, in consequence of their refusing to get wet, and the emery worked up on the side of the jug is also a source of danger, therefore wipe the jug round inside before decanting.

In order to get a uniform grade stop the currents of water in the jug, which may work up coa.r.s.e particles, by holding a thin bit of wood in the rotating liquid for a moment, and then gently withdrawing it in its own plane. These precautions are particularly necessary in the case of grades Nos. 2, 3, and 4, especially No. 4, for if a single coa.r.s.e particle gets on the tool when the work has progressed up to this point it will probably necessitate a return to grinding by means of No. 2, and involve many hours' work.

The surface of the lens will require to be ground continuously with each grade till it has the uniform state of roughness corresponding to the grade in question. Two hours for each grade is about the usual time required in working such a lens as is here contemplated.

The coa.r.s.er grades of emery may be obtained by washing ordinary flour of emery, but the finer ones have to be got from emery which has been used in the previous processes. It is not a good plan to wash the finer grades of emery out of the proceeds of very rough grinding say with anything coa.r.s.er than flour of emery--as there is a danger of thereby contaminating the finer grades with comparatively coa.r.s.e gla.s.s particles (owing to their lightness) and this may lead to scratching.

If the finer grades are very light in colour, it may be inferred that a considerable portion of the dust is composed of gla.s.s, and this does no good. Consequently time may be saved by stirring up the light-coloured ma.s.s with a little hydrofluoric acid in a platinum capsule; this dissolves the finely divided gla.s.s almost instantaneously. The emery and excess of hydrofluoric acid may then be thrown into a large beaker of clean water and washed several times.

Fine emery thus treated has much the same dark chocolate colour as the coa.r.s.er varieties.

The operator should not wear a coat, and should have his arms bare while working with fine emery, for a workshop coat is sure to have gathered a good deal of dust, and increases the chances of coa.r.s.e particles getting between the surfaces.

-- 61. Details of the Process of Fine Grinding.

A lens of the size selected for description is mounted as before mentioned on a leaden pedestal, and the operator places the latter on a table of convenient height in a room as free from dust as possible.

Everything should be as clean as a pin, and no splashes of emery mud should be allowed to lie about. I have found it convenient to spread clean newspapers on the table and floor, and to wear clean linen clothes, which do not pick up dust. I have an idea that in large workshops some simpler means of avoiding scratches must have been discovered, but I can only give the results of my own experience. I never successfully avoided scratches till I adopted the precautions mentioned.

Fig. 52.

The left hand should be employed in rotating the pedestal either continuously (though slowly) or at intervals of, say, one minute.

This point is rather important. Some operators require two hands to work the grinding tool, and in any case this is the safer practice.

Under these circ.u.mstances the pedestal may be rotated through one-eighth or tenth of a revolution every three minutes, or thereabouts. The general motion given to the grinding tool should be a series of circular sweeps of about one-fourth the diameter of the gla.s.s disc, and gradually carried round an imaginary circle drawn on the surface of the lens and concentric with it (Fig. 52).

The tool may overhang the lens by a quarter of the diameter of the latter as a maximum. The circuit may be completed in from twelve to thirty sweeps. The grinding tool should be lightly held by the fingers and the necessary force applied parallel to the surface. The tool itself must be slowly rotated about its axis of figure. If the tool be lightly held, it will be found that it tends to rotate by itself. I say "tends to rotate," for if the tool be touching evenly all over the surface it will rotate in a direction opposite to the direction of the circular sweep. For instance, if the tool be carried round its looped path clockwise, it will tend to rotate about its own axis of figure counter-clockwise. If it touch more in the middle, this rotation will be increased, while if it touches more along the edge, the rotation will be diminished, or even reversed in an extreme case.

Every fifty sweeps or so the tool should be simply ground backwards and forwards along a diameter of the lens surface. This grinding should consist of three or four journeys to and fro along, say, eight different diameters. About one-quarter of the whole grinding should be accomplished by short straight strokes, during which the tool should only overhang about one-quarter of an inch. The object of the straight strokes is to counteract the tendency to a gradual acc.u.mulation of the emery in the centre, which results from the circular grinding.

A great deal of the art of the process consists in knowing how to work the tool to produce any given effect. For instance, if the lens requires to be ground down near the centre, the epicycloidal strokes must be nearly central; the tool must never overhang very much. If, on the other hand, it is the edges which require attention, these must be dealt with by wider overhanging strokes. The tool must be frequently tested on its fellow, and, indeed, ground upon it if any marked unevenness of action (such as that just described) is required for the lens. A check by spherometer will be applied at intervals according to the judgment of the operator, but, in any case, the fellow tool and lens should be kept at very nearly the same figure.

The emery should never be allowed to become anything like dry between the tool and the lens, for in some way (probably by capillary action increasing the pressure of the tool) this seems to lead to scratching and "rolling" of the emery. The channels in the gla.s.s tool between the squares are of the greatest importance in enabling the emery to distribute itself. Perhaps the best guide in enabling one to judge as to when it is time to wash off the emery and apply fresh is the "feel" of the tool; also when the mud gets light in colour we know that it is full of gla.s.s dust, and proportionately inoperative.

New emery may be put on, say, every five minutes, but no absolute rule can be given, for much depends on the pressure of the tool upon the lens. In the case considered a bra.s.s or lead, or even slate tool, of an inch, or even less, in thickness, will press quite heavily enough.

In washing the lens and tool before new emery is introduced, a large enamelled iron bucket is very handy; the whole of the tool should be immersed and scrubbed with a nail-brush. The lens surface may be wiped with a bit of clean sponge, free from grit, or even a clean damp cloth.

When the time comes to alter the grade of emery, a fresh lot of newspapers should be put down, and tools, lens, and pedestal well washed and brushed by the nail-brush. The surfaces should be wiped dry by a fresh piece of rag, and examined for scratches and also for uniformity of appearance; a good opinion can be formed as to the fit of the surfaces by noting whether--and if so, to what degree--they differ in appearance from point to point when held so that the light falls on them obliquely.

It is necessary to exercise the greatest care in the washing between the application of successive grades of emery, and this will be facilitated if the edges of the gla.s.s squares were dressed on a grindstone before they were mounted. An additional precaution which may be of immense advantage is to allow the tool to dry between the application of successive grades of emery (of course, after it has been scrubbed), and then to brush it vigorously with a hat-brush. It sometimes happens that particles of mud which have resisted the wet scrubbing with the nail-brush may be removed by this method.

As my friend Mr. Cook informs me that his present practice differs slightly from the above, I will depart from the rule I laid down, and add a note on an alternative method.

Consider a single lens surface. This is roughed out as before by an iron tool, a rough fellow tool being made at the same time. The squares of gla.s.s are cemented to the roughing tool, and this is ground to the spherometer by means of the counterpart tool. The gla.s.s-coated tool is then applied to the lens surface and grinding with the first grade of emery commenced. The curvature is checked by the spherometer. Two auxiliary tools of, say, half the diameter of the lens, are prepared from slate, or gla.s.s backed with iron, and applied to grind down either the central part of the lens surface or tool surface, according to the indications of the spherometer. Any changes that may occur during grinding are corrected by these tools. The spherometer is accepted as the sole guide in obtaining the proper curvature. A slate backing is preferred for tools of any diameter over, say, 2 inches.

-- 62. Polishing.

After the surface has been ground with the last grade of emery, and commences to become translucent even when dry, the grinding may be considered to be accomplished, and the next step is the polishing.

There are many ways of carrying out this process, and the relative suitability of these methods depends on a good many, so to speak, accidental circ.u.mstances. For instance, if the intention is to finish the polishing at a sitting, the polishing tool may be faced with squares of archangel--not mineral or coal-tar--pitch and brought to shape simply by pressing while warm against the face of the lens. A tool thus made is very convenient, accurate, and good, but it is difficult to keep it in shape for any length of time; if left on the lens it is apt to stick, and if it overhangs ever so little will, of course, droop at the edges.

On the whole, the following will be found a good and sufficient plan.

The gla.s.s-grinding tool is converted into a polishing tool by pasting a bit of thin paper over its surface; a bit of woven letter paper of medium thickness with a smooth but not glazed surface does very well.

We have found that what is called Smith's "21 lbs. Vellum Wove" is excellent. This is steeped in water till quite pliable and almost free from size. The gla.s.s tool is brushed over with a little thin arrowroot or starch paste, and the paper is laid upon it and squeezed down on the gla.s.s squares as well as possible; if the paper is wet enough and of the proper quality it will expand sufficiently to envelop the tool without creases, unless the curvature is quite out of the common.

This being accomplished, and the excess of water and paste removed, the face of the paper is (for security) washed with a little clean water and a bit of sponge, and, finally, the tool is slightly pressed on the lens so as to get the paper to take up the proper figure as nearly as possible. After the polishing tool has been thus brought to the proper figure, it is lifted off and allowed to dry slowly. When the paper is dry it may be trimmed round the edges so as not to project sensibly beyond the gla.s.s squares. The next step is to brush the surface over very carefully with polishing rouge (prepared as is described at the end of this section) by means of a hat-brush. When the surface of the paper is filled with rouge all excess must be removed by vigorous brushing.

Fig. 53.

The tool being placed on the lens, two or three strokes similar to those used in grinding may be taken, and the tool is then lifted off and examined. It will be found to be dotted with a few bright points, produced by the adhesion of gla.s.s at the places of contact. These points are then to be removed in the following manner. An old three-cornered file is ground on each side till the file marks disappear, and sharp edges are produced (Fig. 53). This tool is used as an ink eraser, and it will be found to sc.r.a.pe the paper of the polishing tool very cleanly and well.

The bright spots are the objects of attention, and they must be erased by the old file, and the polisher reapplied to the gla.s.s. A few strokes will develop other points, more numerous than before, and these in turn must be erased. The process is continued till the whole surface of the polishing tool is evenly covered with bright specks, and then the polishing may be proceeded with. The specks should not be more than about one-eighth of an inch apart, or the polishing will be irregular.

The operation of polishing is similar to that of grinding. A reasonable time for polishing a gla.s.s surface is twenty hours; if more time is required it is a sign that the fine grinding has not been carried far enough. The progress of the operation may be best watched by looking at the surface--not through it. For this purpose a good light is requisite. When the lens is dismounted it may be examined by a beam of sunlight in a dark room, under which circ.u.mstances the faintest signs of grayness are easily discernible.

It may be mentioned here that if the surface is in any way scratched the rouge will lodge in the scratches with great persistence, and an expert can generally tell from the appearance of scratches what kind of polishing powder has been employed.

The persistence with which rouge clings to a rough surface of gla.s.s is rather remarkable. Some gla.s.s polishers prefer to use putty powder as a polishing material, and it is sometimes said to act more quickly than rouge; from my rather limited experience I have not found this to be the case, but it may have merits that I do not know of. Is it possible that its recommendation lies in the fact that it does not render scratches so obtrusively obvious as rouge does?

Rouge is generally made in two or more grades. The softer grade is used for polishing silver, and is called jewellers' rouge. The harder grade, suitable for gla.s.s polishing, is best obtained from practical opticians (not mere sellers of optical instruments). I mean people like Messrs. Cook of York. Many years ago I prepared my own hard rouge by precipitating ferrous sulphate solution by aqueous ammonia, washing the precipitate, and heating it to a red heat. The product was ground up with water, and washed to get rid of large particles.

This answered every purpose, and I could not find that it was in any way inferior to hard rouge as purchased. The same precipitate heated to a lower temperature is said to furnish a softer variety of rouge; at all events, it gives one more suitable for polishing speculum metal. Lord Rosso used rouge heated to a dull redness for this purpose.

Rouge, whether made or bought, should always be washed to get rid of grit. I ought to add that not the least remarkable fact about the polishing is the extraordinarily small quant.i.ty of the polishing material requisite, which suggests that the process of polishing is not by any means the same as that of exceptionally fine grinding. Is it possible that the chief proximate cause of the utility of rouge is to be sought in its curious property of adhering to a rough gla.s.s surface, causing it, so to speak, to drag the gla.s.s off in minute quant.i.ties, and redeposit it after a certain thickness has been attained on another part of the surface?