CONTINUING ACTION OF LIGHT.
The action set up by light in a colloid substance when impregnated with chromic salts continues in a less degree even after the substance is removed from the light ; that is to say, a print exposed for a part of the proper time and then kept for a while in the dark before development, would afterwards be found to be fully exposed. It was therefore natural to argue that this hardening effect might be conveyed to a film which had never been in the light at all, and that the image might be formed by one paper and de veloped by means of another. On this basis the various processes under con sideration were founded. This continuous action, it was discovered, proceeded even though there were no free bichromate present in the film. This discovery was of great importance, as it became possible to keep the printed image— or, rather, the basis—for four or five months between printing and developing. Thus the chemical action may be started in one substance, conveyed to another either with or without free bichromate, and the second substance and the one actually holding the pigmented image may differ entirely from the first. For ex ample, the image may be (a) produced straight away on pigmented gum ; or (b) on gelatine only, and afterwards formed in gum and pigment ; or (r) on paper only, and afterwards in gum, starch, or gelatine.
Gum litenrcoarATE PROCESS.
A large variety of methods have been suggested, and there seems to be consid erable latitude in formulte and mode of working ; especially if the smeary indis tinctness, which in some minds has be come associated with the process, is admired. The following method is one of
many:—A sheet of Whatman's drawing paper, or other suitable paper, is sensitised by immersion in a 5 per cent. solution of potassium bichromate, and dried. It is then coated with a mixture of gum and pigment, and exposed beneath a negative, the exposure being timed by means of an actinometer. To develop the print, it should be laid face down in a deep dish of cold water, and allowed to float in this manner for five or ten minutes. The gum and pigment should very gradually soak off the unexposed portions. Development may take hours in sonic cases, but this will depend on the. length of exposure and temperature. It. is best, however, not tc hurry it. At the end of each ten minutes or so it may be examined, and treated in accordance with its condition. Develop ment may be hastened by laving it wit}, water, or in some cases by gentle treat ment with a large camel-hair mop. It extreme cases, it may even be advisable to stipple clown the deep shadows with the brush full of water. All the time the print is face upwards it should have gentle stream of water flowing over it On no account should the print be allowec to remain face up to soak. When de veloped, the print is fixed in a solution o common alum—strength immaterial—an( dried. Instead of alum, either hypo. o, soda sulphite may be used. It has beer claimed for this process that it lends itsel above all others to artistic modifications but for extreme latitude in this direction it cannot equal the dusting-on process (see p. 201).