Either ferrous-oxalate or bydroquinone development may be used, but I decidedly prefer the former for developing chloride plates. But whichever is employed some modifi cation of the original formula will be necessary, as the solutions used for developing negatives will prove far too strong for developing chloride lantern plates.
To make the developer, place in a clean developing cup, Oxalate solution . . 4 parts.
Iron solution . . 1 part.
Water, distilled . . 12 parts.
In using this developer the greatest care must be taken to avoid the slightest contamination with hypo. or gyro., which would be fatal to good results. If the exposure has been correct, the picture will soon begin to appear, and will gain rapidly in strength and detail. The plate must be removed from the developer as soon as the picture is fully out, as the image will continue to gain in density even after it has been placed in the washing water. Density is ascer tained by removing the plate from the dish and holding it up to the light of the lamp. At the close of development the high lights, represented by the white portions of the picture, should remain clear ; if they become discoloured, the exposure has been excessive, or development has been carried too far. In either case the slide should be rejected, as it is of the utmost importance that the high lights of a lantern slide should be transparent and free from the slightest veiling or discoloration. After washing in running water, the plate should be transferred to a freshly-made fixing bath of ordinary strength. These plates fix very quickly, but they should be allowed to remain in the bath for about ten minutes, after which they must be washed in running water for two hours. They may then be re moved, and put on end against a wall in a place free from dust until they are thoroughly dry.
The slides will then be ready for mounting. Some narrow strips of ready-gummed paper, called in technical language " binding strips," should be purchased from a dealer, together with some cover glasses, which are simply plain pieces of glass the size of a lantern plate. Some "masks," as the discs of opaque paper 34 by 34 square with various sized openings punched in them are called, will also be required. The size and shape of the opening chosen for
any particular slide must be left to the taste and judgment of the reader ; but as a general rule oblong openings will be found to yield the most artistic results, and circles should whenever possible be avoided.
To mount the slide, a suitable mask is taken and laid upon the face of the slide, a clean cover-glass is then super imposed, so that the film of the slide is entirely protected from any injury, short of actual fracture. A strip of the gummed binding paper is cut into 34 in. lengths and moistened. On one of these moistened strips the slide with its covering glass is pressed ; it will adhere, and the sides should then be quickly and neatly pressed to the glass. The operation is repeated with the remaining three sides, when the slide will be complete. In order that a lantern slide may appear correctly on the screen, it is necessary to place it in the lantern upside down, and with the filne side of the slide towards the condenser ; and so that mistakes in this respect may not occur, it is usual to place some dis tinguishing mark upon the slide. This usually takes the form of two discs of paper, which should be gummed to the top corners of the slide, when it is held up to the light, and the view appears in its proper position to the observer. In exhibiting, the slides are always inserted in the lantern with these dots down and towards the condenser.
To properly trent the subject of lantern slides it would be necessary to go into far greater detail than would be permissible in these pages ; therefore I have contented my self with describing the simplest method of producing thena, and the easiest process by which they may be produced. Having succeeded in producing successful results by the simple process described, the worker can try his hand at the reduction method, and can experiment with different makes of plates. Full instructions for producing lantern slides by all the most suitable processes will be found in. " The Lantern-slide Manual."*