A Manual Of Photography

14 HISTOBY OF PHOTOGRAPHY. should be held before the mouth, in order to condense the mois- ture of the breath.” The plate thus prepared is now in a fit state for use, and may be immediately fixed in the correct focus of the camera. After it has been exposed a sufficient length of time for receiving the impression, a very faint outline alone is visible. The next opera- tion is to bring out the hidden picture, which is accomplished by a solvent. This solvent must be carefully adapted to the purposes for which it is designed: it is difficult to fix with certainty the pro- portions of its components, but in all cases it is better that it be too weak than too strong ; in the former case the image does not come out strongly ; in the latter it is completely de- stroyed. The solution is prepared of one part—not by weight, but volume—of the essential oil of lavender, poured upon ten parts, by measure also, of oil of white petroleum. 1 he mixture, which is first milky, becomes clear in two or three days. This compound will act until it becomes saturated with the asphaltum, which state is readily distinguished by an opaque appearance, and dark brown colour." A tin vessel somewhat larger than the pho- tographic tablet, and one inch deep, must be provided. This is to have as much of the solvent in it as will cover the plate. The tablet is plunged into the solution, and the operator, observing it by reflected light, begins to see the images of the objects to which it has been exposed slowly unfolding their forms, though still veiled by the gradually darkening supernatant fluid. The plate is then lifted out, and held in a vertical position, till as much as possible of the solvent has been allowed to drop away. When the dropping has ceased, we proceed to the last, and not the least important operation, of washing the plate. This is performed by carefully placing the tablet upon a board, b, fixed at a large angle, in the trough A A, the supports being joined to it by hinges, to admit of the necessary changes of inclina- tion, under different circum- stances : two small blocks, not thicker than the tablet, are fixed on the board, on which the plate rests. Water must now be slowly poured upon the upper part of the board, and allowed to flow evenly over the surface of the picture. The descending stream clears away all the solvent that may yet