Time of Exposure.--With regard to the time which should be allowed for the paper to remain in the camera, no direct rules can be laid down; this will depend altogether upon the nature of the object to be copied, and the light which prevails. All that can be said is, that the time necessary for forming a good picture varies from thirty seconds to five minutes, and it will be naturally the first object of the operator to gain by experience this important knowledge. Bringing Out the Picture.--The paper when taken from the camera, which should be done so as to exclude every ray of light--and here the dark slide of the camera plate holder becomes of great use--bears no resemblance to the picture which in reality is formed. The impression is latent and invisible, and its existence would not be suspected by any one not acquainted with the process by previous experiment. The method of bringing out the image is very simple. It consists in washing the paper with the gallo-nitrate of silver, prepared in the way already described, and then warming it gently, being careful at the same time not to let any portion become perfectly dry. In a few seconds the part of the paper upon which the light has acted will begin to darken, and finally grow entirely black, while the other parts retain their original color. Even a weak impression may be brought out by again washing the paper in the gallo-nitrate, and once more gently warming it. When the paper is quite black, as is generally the case, it is a highly curious and beautiful phenomenon to witness the commencement of the picture, first tracing out the stronger outlines, and then gradually filling up all the numerous and complicated details. The artist should watch the picture as it developes itself, and when in his judgment it has attained the greatest degree of strength and clearness, he shall stop further proceedings by washing it with the fixing liquid. Here again the mixed solution need not be used, but the picture simply brushed over with the gallic acid. The Fixing Process.--In order to fix the picture thus obtained, first dip it into water; then partly dry it with bibulous paper, and wash it with a solution of bromide of potassium--containing one hundred grains of that salt dissolved in eight or ten ounces of distilled water. The picture is again washed with distilled water, and then finally dried. Instead of bromide of potassium, a solution of hyposulphite of soda, as before directed, may be used with equal advantage.