A Manual Of Photography

tllE DISCOVEBY 0E M. DAGtEBBE. 39 tit which this effect is produced, a violet colour is assumed, which is much less sensitive to light; and if the yellow,coating is too pale, the picture produced will prove very faint in all its parts. The time for this cannot he fixed, as it depends entirely on the temperature of the surrounding air. No artificial heat must be applied, unless in the case of elevating the temperature of an apartment in which the operation may he going on. It is also important that the temperature of the inside of the box should be the same as it is without, as otherwise a deposition of moisture is liable to talte place over the surface of the plate. It is well to leave a portion of iodine always in the box ; for, as it is slowly vaporized, it is absorbed by the wood, and when required it is given out over the more extended surface more equally, and with greater rapidity.. As, according to the season of the year, the time foi pro- ducing the required effect may vary from five minutes to halt an hour, or more, it is necessary, from time to time, to inspect the plate. This is also necessary, to see if the iodine is acting equally on every part of the silver, as it sometimes happens t låt the colour is sooner produced on one side than on the other, and the plate, when such is the case, must be turned one quarter round. The plate must be inspected in a darkened room, to which a faint light is admitted in some indirect way, as by a door a little open. The board being lifted from the box with both hands, the operator turning the plate towards him rapidly, observes the colour. If too pale, it must be returned to the box ; but if it has assumed the violet colour it is useless, and the whole process must be again gone through. From description, this operation may appear very difficult; but with a little practice the precise interval necessary to pro* one half of the circle with a card, and expose the other part to light, the rings will be found to change colour, the outer and the inner yellow dark- ening the most readily, and to an equal shade ; thus proving the advantage of obtaining this yellow tone.