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64 The Audio-Visual Handbook
Lantern-Slide Plates. The Eastman or any other "slow" plate is the type which will prove to be most satisfactory for the beginner, as it is less sensitive to light than other types. The plate is a piece of clear glass, 3Vix4 inches in size, one side of which has been sensitized by coating it with an emulsion composed of silver bromide and silver iodide dissolved in gelatin. The emulsion side is the dull side of the slide. The slides are packed in boxes of twelve and the boxes should be opened only in a darkroom. Furthermore, it is important that the boxes be kept tightly closed at all times except when the room is thoroughly dark or illuminated only by the red light.
Negatives. Any negative which will make a good print will make a good lantern slide. The lantern slide is merely a positive print of the negative used. When making contact prints, it is necessary that the negative be approximately 2^4x3 inches in size, or that the essential part of the negative to be transferred to the glass slide be confined to those dimensions. If materials are to be photographed for transfer to glass slides, it will be well to use a camera which accommodates a negative 21/4x31/4 inches in size. Any size of picture may be transferred to a lantern slide, of course, but the use of the smaller negative will tend to eliminate one or two processes. There are lantern-slide negatives on which drawings, photographs, etc., may be recorded, and these same negatives may be used over and over again in the production of additional slides of those subjects. The beginner should learn the simpler processes first, however, and the more difficult procedures may be accomplished later.
Contact Printing. As mentioned above, this is a very easy method for the beginner, and very satisfactory. Simply place the emulsion or dull side of the lantern-slide plate against the dull side of the negative from which the picture is to be made and hold each against the other firmly by using an ordinary printing frame, with the plate below the negative. The 25-watt frosted tungsten lamp should be suspended about six feet above the workbench and, after the negative and plate have been fixed firmly in place, should be snapped on for about fifteen seconds. Turn out the white light and continue with the developing process, using only the ruby light.
Developing. One of the simplest developing solutions is prepared by using "Nepera," or any other developer in which the various chemicals are ready mixed. This developer is available in eight or sixteen ounce bottles and is inexpensive.
Mix up about one-half pint of the solution, using one part of