AS a laboratory experiment this is an extremely interesting process, but from the practical point of view it is not worth wasting an hour over, in the present state of our knowledge. It was originally suggested by Ducos du Hauron and Chas. Cros from purely theoretical reasoning and lay dormant from that time, 1867, until 1895, when it was put into practical use. The principle of the process is based on what is known as the Grotthus-Draper law, which is, that only light which is absorbed can exert chemical action; this may be extended and amplified by stating that, as only complementary colors are absorbed, it is only these that can act on a colored substance.

Briefly put, three fugitive aniline dyes, red, yellow, and blue, are mixed together in suitable proportions, so as to give a neutral grey tint. If such a film be exposed to white light they will all fade and we shall see the white support; but if exposed to colored light, then the limitation already pointed out as to the complementary colors comes into play. Under a red glass, the blue and yellow dyes fade, as together they make green; under green glass, the red fades, as this is complementary to green; under the blue, the yellow and red fade out, leaving only the blue. The action of any intermediate color is explicable in the same way, black being, of course, formed by none of the dyes fading. It is clear, then, that we must start with a colored original, and for purely experimental work, nothing is better than pieces of stained gelatine bound up with glasses.

A great deal of experimental work has been done on this process, and bleach-out paper was for a few years a commercial article. Unfortunately, while great advances have been made in the rapidity of the bleaching, it has not yet been possible to find any real fixing agent, that is, any substance that will prevent the dyes, forming the image, from still further bleaching. So the results, when exposed to white light, gradually fade out.

Exact formulas cannot be given, as so much depends on the dyes; but sufficient may be told to lead the experimenter on the right road. Unfortunately, the preparation of the paper direct is not satisfactory, as the dyes have a tendency to wander or migrate into the paper itself, so that most of the best results have been obtained by coating on opal glass, whence the film is stripped. If it is not desired to use this, then gelatinized paper may be used; this can be prepared, as has already been suggested, by fixing out bromide or other paper, and hardening. The plan already proposed of stretching the paper may also be adopted.

To prepare the dyed gelatine, the following solutions should be made up: methylene blue BB, 2 per cent solution in water; auramin concentrated, 2 per cent solution in alcohol; bluish erythrosin, 0.5 per cent solution in water. Soft emulsion gelatine and hydrogen peroxide will also be required. There are two methods of preparing the paper; either the paper is immersed in an ethereal solution of hydrogen peroxide, or an aqueous solution of the peroxide is used to make the gelatine solution. Dealing with the former method first, prepare a solution of:

Soft gelatine 100 g.

Distilled water 1000 ccm.

Allow the gelatine to soak for about 15 minutes, then melt by the aid of a water bath, and divide into three portions. To one, add 40 ccm of methylene blue solution; to the second lot add 20 ccm of the auramin solution, and to the third 15 ccm erythrosin solution. Then add the yellow solution to the blue, and finally, with constant stirring, gradually add the red solution. Towards the end this red solution must be added very gradually, and after each addition, a drop of the mixture should be placed on white paper. As soon as a faint reddish tinge is apparent, the addition of the red must be stopped; the main color must be grey.

Now keep the dyed gelatine at a temperature of about 40 ° C. (104 ° F.) for from four to five hours; then filter, and it is ready for coating. If the paper has been stretched on glass it can be treated exactly like a plate which is to be collodionized, that is, supported on the back with a rubber bulb, the dyed gelatine poured on and the plate inclined until the surface is covered, and then put on a level slab to set. If it is to be coated by floating, then the gelatine must be poured out into a dish, any air bubbles broken or led to the sides, the paper bent into the form of a J and the bottom of the loop carefully lowered on to the surface at one end of the dish, with the short end of the loop pointing towards the middle of the dish, and the other end slowly lowered and the paper allowed to slide along the surface of the gelatine. With a little practice this can be done without forming any air bubbles. As soon as the paper begins to curl at the edges, it should be slowly drawn off the surface and immediately turned over on to a flat plate to set. The temperature naturally plays a great part in determining the quantity of gelatine that adheres to the paper; the lower it is, the thicker the film. From 35 ° to 38 ° C. (95 ° to 100 ° F.) is a good range. If the first coating does not give an even coat, the paper may be floated again when it is dry. The paper should be dried in the dark.

To sensitize this paper with the ethereal solution, the latter must be made by shaking 15 ccm of 30 per cent peroxide solution with 200 ccm ether for about ten minutes and allowing to settle in a burette or funnel; the water is then drawn off. This forms a one per cent solution of peroxide in ether. It must be noted that the peroxide solution is not the household one, which only contains three per cent of peroxide; the proper one is ten times the strength, and is sold as "perhydrol."