PRESERVATION OF THE PAPER.
Gelatine is a substance somewhat liable to be affected by surrounding conditions ; that is to say, it is liable to decomposition if stored in a moist atmosphere. This danger is. increased if the air is warm, as . will have been gathered from the previous remarks on gelatine. The air should be as pure as possible, and for this reason the paper should not be stored in proximity to volatile chemicals. Albumen paper, which as it becomes old gives off sulphur etted hydrogen, is liable to cause deterior ation of a bromide paper if kept too near it. The fumes of gas, oil, ammonia, etc., are all destructive, and the paper should, therefore, be placed on a high shelf, neither should it be stored on the floor. (These remarks apply equally to other pcmulsion papers and plates also.) When the supply kept is small, and is quickly used up, such matters need not trouble the user ; but workers abroad, and those who have to keep a quantity of paper in stock, will do well to give this serious attention. A suitable cupboard on a level with the body, will be very serviceable, if kept en tirely for emulsion paper and plates. The paper is best when freshly prepared, as the tendency. with emulsions is to become less sensitive after long keeping, thereby giving harder contrast. The best plan in
dealing with an opened package of paper, made use of, as the negative may be used straight away without waiting for it to dry, and even, in some cases, without wait ing for it to fix. If the source of light is small, a very sharp shadow is cast, which means that it is not so essential that the paper and negative should be in absolute contact as when a diffused light like day light is used. An experiment will illustrate this. Set up a focussing screen at A (Fig. 314) and a cross cut in cardboard n close to it, and at c place. a lighted candle. Move a away from A say of an inch, and its outlines still remain clear although the shadow thrown is a little larger. Now place a sheet of ground glass in front of a so that the light passes through it, and is to place it face down in a box—an or dinary dry plate box—and lay over it a sheet of stout plate glass, which by pres sure excludes the air. Paper is usually packed in a proper condition for keeping when sent from the factory, so that it need not be interfered with. It is an advan tage, under some circumstances, if the package is wrapped in waxed paper or tinfoil