A Manual Of Photography

248 PRACTICE OF PHOTOGRAPHY. if not level, it must be adjusted: the level will be easily seen through the glass slide. When everything is thus arranged, the plate, previously iodized, is to he placed in its frame over the pan, the slide withdrawn, and the necessary time counted; after this has elapsed, the slide should be shut, and the plate immediately placed in the dark box of the camera. For a second operation, this bromine water must be thrown away, and a fresh quantity used. The bottle containing the bromine water should be kept away from the direct light of the sun, and care should be taken that no organic matter fall into the bottle, such as grease, chips of cork, &c. These enter into new combinations witli the bromine, and lead to error as to its amount in solution.. Daguerre himself introduced some very considerable improve- ments in the process of iodising. He avoids the use of metal strips, and gives some curious experiments on the action of edges, grooves, &c., in determining the deposition of vapour. M. Daguerre then states that, but for the difficulty of fixing them, the bands might be very much reduced in size ; for it is sufficient for them to produce their effect that there be a solution of continuity between them ; and this is proved by the fact that nearly the same result is obtained by engraving at the »th of an inch from the edge of the plate a line deep enough to reach the copper. The objections to this are, that during the polishing process the engraving is filled witli dust, and it retains water, which sometimes occasions stains. He then proposes, as a very great simplification of this process, that the plate be laid flat in a shallow box containing two grooves, one to receive the plate, and the other a board saturated with iodine. Around the plate he places a border of either powdered staroli or lime, and the iodine descends from the board to the tablet. The starch or lime absorbs the iodine with avidity, and thus prevents its attacking the edges of the silver, and the vapour is diffused with perfect evenness over it. Another advantage is, that the satu- rated board may be used for several days in succession, without being at all renovated. M. Seguier somewhat modifies even this process. A box oi hard wood, varnished internally witli gum lac, contains a lump of soft wood, furnished with a card of cotton sprinkled with iodine Upon this is placed a plate covered with card-board on each of its faces. One of these card-boards furnishes, by radi- ation, to the metal the vapour of iodine, while the oilier returns to the cotton that which it had lost. It suffices to turn the plate from time to time, in order that the operation may go on with equal rapidity. A plate of glass is placed upon the upper