Development of the latent impressions occurs rapidly when the specimen is exposed to a blue or violet light source. A 1,000-watt blue or daylight photographer's lamp, a mercury arc (most ultraviolet lamps are of this type), or carbon arc is excellent for the purpose (fig. 422). If a weaker light is used, a stronger mixture of the solution should be prepared. For instance, if a 300-watt bulb is used, the 10-percent solution would be preferable. Direct sunlight will cause the latent impressions to appear very rapidly and if several specimens are exposed at once it is not possible for a single operator to properly control the development. Sunlight coming through a window pane will serve for development. Where fingerprints containing sodium chloride (normally exuded from the sweat pores in the ridges) have been deposited, the silver chloride formed will darken against the background. [Illustration: 422. Developing silver nitrate prints using 1,000-watt bulb reflector.] As soon as the ridge detail of the prints is clearly visible, the paper should be removed from the light. Continued exposure will darken the paper and the contrast will be lost. Paper so treated should be kept in darkness; that is, in a heavy envelope or drawer until ready to photograph. Immediate photographing, as in the case of iodine prints, is not always essential, since the prints are permanent and become illegible only through eventual clouding of the background. Prompt photographing is recommended, however, as, in exceptional instances, silver-nitrate prints have become illegible in a matter of hours. Darkening ordinarily will occur slowly if the paper is preserved in absolute darkness, and silver-nitrate prints so preserved more than 10 years have been observed to be quite clear. Items such as cardboard cartons, newspapers, road maps, large pieces of wrapping paper, or smooth, unpainted wood surfaces, too large for dipping, may be treated by brushing the solution over the surface with a paint brush (fig. 423). Brushing does not damage or destroy latent impressions on surfaces of this type. Cardboard boxes may be slit down the edges and flattened out to permit easy placement under the light. [Illustration: 423. Silver nitrate solution being applied with paint brush.] Wet paper should be handled with extreme care to prevent tearing. In treating very thin types of paper the solution is best applied with a cotton swab or brush. Photographs, Photostats, and blueprints of any value should not be treated with silver nitrate, since the developed prints or stains cannot be removed without destroying them. In working with silver nitrate, wear rubber gloves or handle all specimens with tweezers; avoid spilling it on clothing. It will cause dark brown stains on clothing, skin, and fingernails. Such stains are not easily removed. Areas of the skin subjected to prolonged contact are deadened, will turn black and peel.