| Alcohol | 1 | part. |
| Ammonia | 1 | drachm. |
until all the varnish is removed, then rinse in alcohol, wash well under the tap, dry, paste slips of brown paper around the margin, and finally level and cover with the warm gelatine, dry and strip.
THE REVERSING MIRROR.
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.