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Alpha

exposure, paper, gelatine and developer

ALPHA PAPER.—A name given in England to a printing paper coated with a gelatine film, in which is contained a combination of silver with a chloride and a bromide. It is used for obtaining positives by artificial light, and is subjected to much the same treatment in develop ment as bromide paper (see Bromide Paper). An exposure somewhat more protracted is, how ever, necessary, owing to the presence of a chloride in the film. The same amount of careful attention and cleanliness is, however, necessary to obtain good results. The following formula • has been given for the preparation of the emulsion : Allow the gelatine to swell in cold water for thirty minutes, heat to Iso degs. Fahr., add salts and acid, and then in the dark-room pour Solution No. 1 into No. 2 always. Constantly stirring, add to the mixture, while still hot, Nelson's gelatine 100 and Heinrich's gelatine TOO grains, previously swollen in water. When dissolved, pour into a flat dish, cut up into small pieces, or squeeze through netting, and wash as in an ordinary emulsion. Remelt and coat the paper. For methods of washing emulsions, see Emulsion, and for coating the paper, see Gelatino-Chloride Printing-Out Process.

The ferrous-oxalate developer gives the best tones, but, if the clearing bath be used, an alkaline developer may be employed. After developing, the prints should be well treated with an alum and acid bath to clear away all yellowness ; then they may be toned in a gold or platinum bath. After that the usual method of fixing, varied in length of time in certain cases, according

to the color that is required, warm or cold ; finally, a thorough washing in several changes of water. They may afterward be allowed to dry spontaneously, with a dull surface, or given a polish in the same manner as recommended for Aristotype Prints. The favorite oxalate developer is the following : Instead of toning, a variety of other colors can be obtained by development. Brown tones are produced by adding common salt and citric acid to the developer ; red prints by the addition of ammonia. Other colors can be obtained by variation in the exposure and restraining develop ment.

ALTISCOPE.—An optical instrument having an elevated, inclined mirror, by the aid of which it is possible to look over near objects standing in the line of vision.

ALTITUDE.—Altitude is an important although barely recognized factor in the calcula tion of exposure. Bunsen discovered that the actinicity of the light increased proportionately with the increase of the altitude above sea level, and Professor Ehrmann has pointed out* that, at Colorado, an exposure meter proved an absolute failure on account of the high situation of that place. At a spot x,5oo feet above the level of the sea, two seconds' exposure was sufficient to expose a plate that, with the same conditions at sea-level, required at least three seconds.