Apparatus

A deep porcelain vessel, about 1/2 in. larger all round than the mirror and preferably with a flat bottom. A 4 oz. measure, and one of 1/2 oz. graduated in minims. Two or three large glass or porcelain jugs to hold about a quart. Cotton wool.

I will describe the silvering of the 9 in. mirror:

First clean the mirror all over, back and sides as well, with a litle alcohol and rub with cotton wool; then rub well with the bichromate solution and let it soak in the same for ten minutes or so. Repeat the cleaning with bichromate until, when wetted, the film of water adheres to every part of the surface. Too much care cannot be taken over this, as unless the mirror is quite clean, the silvering is likely to be a failure. Rinse the mirror well under the tap and finally with distilled water. Place it in the silvering vessel face up. Pour out, for a 9 in. mirror, a pint of dis tilled water into the silvering vessel which, with all measures, jugs, etc., must have been previously cleaned with bichromate solution and distilled water. This will cover the mirror to the depth of, perhaps, 1/4 of an inch.

Then measure out 320 minims, or 5 dr. 20 min., of formalin and add to the water. Rock the silvering vessel well so that the formalin and water are properly mixed.

Then take 2 oz. of the 10 per cent nitrate solution and slowly add the ammonia solution. A dense brown precipitate is at once formed; as the ammonia is added this will gradually disappear and the addition of ammonia must be stopped just before the solution clears. If any excess of ammonia is allowed to get in a little more nitrate of silver must be added, so that the solution is slightly clouded. The measure should be well rocked during the addition of the ammonia, and great care taken not to have any excess. The slight cloudiness left does no harm but an excess of ammonia means almost certain failure. The ammoniated solution is now poured into a jug and a pint of distilled water added. This is poured rapidly into the silvering vessel, and over the mirror, and the vessel well and vigorously rocked.

The action is extremely rapid. In a very few seconds the solution begins to turn pink, and a black deposit appears on the mirror - the solution darkens to brown and then black and the mirror will, if all is well, be covered completely with a deposit of silver before the solution has lost its transparency. The rocking is continued until the action has ceased, when the solution will become transparent, but filled with black coagulated particles. The mirror is then lifted out - no fear need be entertained of staining the fingers at this point - and rinsed, first with distilled water and then under the tap for a few minutes. The film should be nearly, if not quite, opaque to the light from the window, although if the film is a little thin it will still perform well. The mirror is placed on its edge on a pad of blotting paper and leaning against some support in a place as far as possible free from dust. It must be allowed to become absolutely dry before any attempt is made to polish.

To polish the film, prepare two pads of the finest chamois leather by tying up a handful of cotton wool in a piece of leather about 8 in. square. See that these are quite dry; they may be warmed in front of a fire with advantage.

When the film is quite dry it is rubbed all over with one of the pads, in small circular strokes, with little pressure, until any bloom on deposit has disappeared. The film is usually fairly brilliant on leaving the solution ; but to get the best possible poi sh it is, as a rule, necessary to use a little rouge. The other pad, therefore, is dipped in a little of the finest rouge and well rubbed on a glass plate. It is then used to polish the mirror in a similar manner, being rubbed all over it with circular strokes and slight pressure. If it is found that the film is rubbed away, it is a sign that it was left rather too long in the silvering solution - the exact moment at which it should be taken out being a matter of experience.

The quantities given above are for a 6 in. mirror 1 1/2 in. thick, in a vessel 1 1/2 in. larger all round than the mirror. The quantity required varies, of course, with the size of the mirror, but should be sufficient to cover the mirror to a depth of at least 1/2 inch. Each ounce of the solution contains 24 minims of the nitrate solution, or about 2-2 grains of nitrate, and 8minims of formalin.

The quantity of formalin given can only be taken as approximate. It varies with the temperature and with the particular sample of formalin employed. It is always a good plan to make a few experiments on small pieces of glass, or even on the inside of a test tube or measure. Too much formalin causes the action to be too quick, and the silver coagulates in the solution before it has had time to deposit on the mirror; too little formalin causes the action to be too slow or to fail altogether.

The temperature of the silvering bath should not be less than 65° or 70°, and may be even higher with advantage. I have got good film with the temperature as low as 48° F.; but it is not easy.

There is no difficulty in silvering the mirror face down if it is thought better, the mirror being supported by pitch on a cross-piece of wood; but I got perfectly satisfactory results by the method given above. It is an advantage to divide the solution into two parts, as above; if the solution is mixed in a jug and then poured on, the action begins so rapidly that an uneven film often results.

Considerable economy of solution may be effected by making the mirror serve as its own silvering dish. For this purpose the edge thereof should be fairly smooth. A band of stout brown paper 3 or 4 inches wide and long enough to go two or three times around the mirror, is dipped in melted paraffine and stretched around the edge of the mirror while still hot, and tied or strapped tightly. In this way a dish 1 1/2 to 2 in. deep is formed, the bottom of which is the mirror itself, and in this way I have obtained a good film with only 6 oz. of solution. The silver film, once in use, should be left alone as much as possible, all that is necessary generally being to dust the surface lightly with cotton wool from time to time. Any violent changes of temperature, which would produce dewing, should be avoided as much as possible, and the telescope, when not in use, kept in a porch or shed, but not indotrs, where the temperature is higher than outside. The silvering process given above, when once mastered, is so quick and handy that it is virtually no trouble.