Читать книгу Photo-engraving, Photo-etching and Photo-lithography in Line and Half-tone - W. T. Wilkinson - Страница 17

The mirror is a piece of perfectly plane glass, coated on the surface with a thick film of pure silver, and highly polished. This silvered glass must be mounted in a mahogany box, and fitted on the front of the camera, the lens being fitted in front of the mirror, see Fig. 1. This mirror is placed in grooves at an angle of forty-five degrees from the axis of the lens. So situated, it receives the image projected by the lens, and reflects it on the focussing screen, or sensitive plate in the camera. By this means a negative is obtained, which, when looked through with the film between the operator and the glass, will present the image in its proper position, whereas, if the lens be used without the intervention of the mirror it will be necessary to have the glass between the eyes and the film, to get the image in its proper position, and as the generality of photographic negatives are taken without the intervention of the mirror, they are called ordinary negatives in contradistinction to those made through the mirror, which are called “reversed” negatives.

The silver surface of the mirror requires great care and attention to preserve its lustre. If tarnished, it will make the exposure in the camera longer, besides which, the cost of resilvering is too great to allow of the surface being spoiled through carelessness. {36}

At the end of each day’s work, remove the mirror from the box and warm it in front of the fire (not over a gas flame) just slightly, then wrap it carefully in a piece of fine velvet, which has also been previously warmed, then wrap it up in a piece of India-rubber or macintosh cloth, and put it in an air-tight box; by doing this, the mirror, if well silvered at first, will last for a year or two.

If the surface should become tarnished, get a square of very fine chamois leather, and place in the centre a pellet of cotton-wool; then gather up the leather and tie the wool in the centre, making a small globe about an inch and a half in diameter; now warm the mirror, and after dipping the leather globe into fine rouge, proceed to polish gently with a quick circular motion, using little or no pressure—take care in doing this that the mirror, the leather, and the rouge are quite dry, else the silver coat will come away—verb sap.

In purchasing a mirror be sure and get it large enough to take the cone of rays from the lens it has to be used with; for a lens three inches in diameter, the plane mirror should measure about 8 × 3 ¹⁄₂ inches.

The box to hold this mirror should be made square at the side, fitting into the rabbet of the camera front, so that when objects are to be photographed that will do better lying on the ground than when fixed against the wall, the mirror can be placed to look down upon the object.

For silvering the glass, one cannot do better than to quote the directions for working Common’s process given by Major Waterhouse in the Photographic News.

“The solutions recommended by Mr. Common are three:

(1)	Nitrate of Silver	1	ounce.
	Distilled Water	10	ounces.
(2)	Caustic Potash	1	ounce.
	Distilled Water	10	ounces.
(3)	Glucose	1⁄2	ounce.
	Distilled Water	10	ounces.

The above quantities are suitable for 250 square inches, consequently, an ordinary copy mirror 8 × 6 would require rather more than two ounces of each solution, and other sizes in proportion.

The caustic potash and distilled water must be quite pure. Ordinary caustic potash will not answer at all. The best to use is known as pure by alcohol.

The glass surface to be silvered is carefully cleaned with strong nitric acid, applied, as recommended by Mr. Browning, with a Buckle’s brush, then well {37} washed in clean water, and after rinsing with distilled water, laid, face downward, in a dish of distilled water until wanted.

Before cleaning the glass, it will be necessary to arrange for supporting it face downward in the depositing dish, so that the surface to be silvered may be quite horizontal, and just below the level of the fluid, which should be about half an inch above the bottom of the dish.

I have generally used a large cork, about four inches in diameter, cemented to the back of the plate, and fitted with three strings, by which it could be suspended in a level position and adjusted to any height by winding the string over a roller placed at a convenient height above the dish. When this arrangement is not available, I fix on the back of the plate two ordinary wide-mouthed bottle corks of equal thickness, in the positions shown in the figure, and to these corks attach thin slips of bamboo running transversely across the plate, and of sufficient length to rest on the sides of the dish, thus:

Fig. 3.

The slips of bamboo give the arrangement a certain amount of spring, by which the height of the plate can easily be regulated, by putting on weights until the surface of the plate is just below the level of the fluid in the dish.

To prepare the silvering solution: A sufficient quantity of the silver solution, No. 1 (two ounces), is put into a perfectly clean glass. Ammonia is dropped in until the precipitate first formed is just redissolved. The same quantity of potash solution, No. 2, as of silver is now mixed in, and the precipitate again dissolved by ammonia. A little more silver solution is then added to produce a distinct turbidity, and distilled water to make up the quantity necessary to fill the depositing dish to about three-eighths or half an inch, and the mixture is then filtered through cotton into another clean glass vessel.

The same quantity (two ounces), of filtered solution of glucose, No. 3, as was taken of silver and potash, is now mixed in, and the whole is poured into a depositing dish (which should preferably be of glass, well cleaned with nitric acid).

The glass plate is then taken out of the distilled water and laid face downward on the silvering solution, being supported, as before described, just above the surface, so that the solution does not cover its back.

Mr. Common places the requisite quantity of distilled water in the dish in {38} which the mirror has been remaining face downward, and then, having lifted the mirror up, pours in the undiluted silvering solution, together with the glucose solution, stirs well together, and then carefully lowers the mirror again into the dish.

Almost immediately after the immersion of the plate, the silvering action begins, and, if things are going on well, a brilliant reflecting surface will be seen at the back of the plate, and in forty minutes, or even less, a good deposit of silver will be obtained. It is usually recommended to stop the action as soon as the silvering fluid appears clear and free from turbidity, but it is not always easy to see this.

After silvering, the plate is well washed, finishing with distilled water, and dried off quickly. A slight cloudiness of the surface may appear, and must be removed by polishing before the mirror can be used. It is better to allow the mirror to remain a day or so before polishing, in order to harden the coating.

To polish the plate, it should be slightly warmed, and perfectly dry, and rubbed very gently in small circles with a piece of very soft and dry chamois leather, afterward using a little jewellers’ rouge.

Mirrors should always be kept in a dry place, and will require repolishing from time to time.