A Manual Of Photography

38 HISTORY OF PHOTOGRAPHE. Second Operation.—It is necessary for this operation, which is really the most important of all, that a box, similar to figs. 4 and 5, be provided. Figure 4 represents a section, supposed to pass down the middle of the apparatus by the line A B in fig. 5, which represents the box as seen from above, c is a small lid which accurately fits the in- terior, and divides the boxes into two chambers. It is kept con- stantly in its place when the box is not in use ; the purpose of it being to concentrate the vapour of the iodine, that it may act more readily upon the plate when it is exposed to it. n is the little capside in which the the iodine is placed, which is covered with the ring, j, upon which is stretched a piece of fine gauze, by which the particles of iodine are prevented from rising and staining the plate, while the va- pour, of course, passes freely through it. E is the board with the plate attached, which rests on the four smaller pro- jecting pieces, R, fig. 5. G is the lid of the box, which is kept closed, ex- cept when the plate is removed or inserted, n represents the supports for the cover c. K, tapering sides all round, forming a funnel-shaped box within. To prepare the plate:—The cover c, being taken out, the cup, 1>, is charged with a sufficient quantity of iodine, broken into small pieces, and covered witli the gauze, j. The board, E, is now, witli the plate attached, placed face downwards, in its pro- per position, and the box carefully closed. In this position the plate remains until the vapour of the iodine has produced a definite golden yellow colour, nothing more nor less.1 If the operation is prolonged beyond the point 1 If a piece of iodine is placed on a silver tablet, it will speedily be sur- rounded witli coloured rings: these being the colours of thin films, as described by Sir Isaac Newton. Close examination will show the formation of two yellow rings, one within and the other without the series. If we cover