A Manual Of Photography

THE USE OF ALBUMEN ON GLASS PLATES AND ON PAPEE. 279 tion of iodine (in a subsequent process) in those than in the surrounding parts. “ On the clear glass pour the albumen, inclining the plats from side to side until it is covered ; allow the excess to run off at one end of the corners, keeping the plate inclined, but nearly vertical. As soon as the albumen ceases to drop rapidly, breathe on or warm the lower half of the plate; the warmth and moisture of the breath will soon cause it to part with more of its albumen, which has now become more fluid: of course, care must be taken to warm only the lower half. "Wiping the edges con- stantly hastens the operation. Until this plan was adopted, the coatings were seldom uniform; the upper half of the plate retained less than the lower. When no more albumen runs down, dry the plate by a lamp, or by a common fire, if the dust that it is inclined to impart be avoided. “The next operation is to iodize the plate. Dilute pure iodine with dry white sand in a mortar, using about equal parts of each ; put this mixture into a square vessel, and place over it the albuminized plate, previously heated to about 100° Fah. As soon as the film has become yellow in colour, resembling beautifully stained glass, remove the plate into a room lighted by a candle, or through any yellow transparent substance, yellow calico for instance, and plunge it vertically and rapidly into a deep narrow vessel containing a solution of one hundred grains of nitrate of silver to fifty minims of glacial acetic acid, diluted with five ounces of distilled water. Allow it to remain until the transparent yellow tint disappears, to be succeeded by a milky- looking film of iodide of silver. Washing with distilled water leaves the plate ready for the camera. “ It may be here noted that the plate is heated in iodizing for the purpose of accelerating the absorption of the iodine: an ex- posure to the vapour for ten minutes, with a few seconds’ im- mersion in the silver solution, has been found to be sufficient.” Hydrochloric acid, chlorine or bromine, may be used with the iodine to give increased sensibility to the plate. The plate is removed from the camera, and we pour over it a saturated solution of gallic acid. A negative Talbotype image is the result. At this point previous experimentalists have stopped. We have gone further, and find that by pouring upon the surface of the reddish brown negative image, during its de- velopment, a strong solution of nitrate of silver, a remarkable effect is produced. The brown image deepens in intensity until it becomes black. Another change commences—the image be- gins to grow lighter; and finally, by perfectly natural magic, black is converted into white, presenting the curious phenomenon of