Bleach-out Process.—A. Vogel (Schweigger's Journal, 1813, pp. 229-236) recorded the fact that under coloured glasses the colours of tinctures of certain flower petals were bleached, but that under glasses of the same colour as the tinctures there was no change. This subject was studied by Herschel (Philos. Trans., 1842) and Robert Hunt (Researches on Light, 1844, p. 17o), and the former propounded the law that dyes were bleached when exposed under their complementary colours, but were not bleached when exposed to light of the same colour. In 1889, R. E. Liesegang (Phot. Archiv., No. 633, p. 328) recommended the admixture of three fundamental colours—red, yellow and blue—and in 1891 (Phot. Almanach, 1891) he stated that the bleaching took place more rapidly in oxygen. Two years later (Phot. Archiv., 1893, Nos. 729, 73o) he also published a series of experiments on the increase of rapidity of bleaching aniline dyes by the addition of certain sensitisers, such as stannous chloride, oxalic acid, hydroxylamine, ammonium sulpho cyanide, etc.
Wiener, in his already cited paper (Annalen d. Phys. u. Chem., 1895, p. 225), includes a special chapter on " the theoretical basis of a method of colour photography with body colours," and says: " In order that a substance sensitive to light can be chemically changed by the action of any kind of light, it must absorb it." The converse proposition is not general. The absorbed light can, for example, be exclusively formed into heat. A distinction is, therefore, made between thermal and chemical absorption of light. For the sake of simplicity of expression I shall designate as a regularly absorbing light sensitive substance one which is sensitive to all colours which it absorbs, and is affected by each colour in proportion to the capacity for absorp tion. That there are such substances, at least to a considerable degree of approximation, is known. Upon their existence is based the important law of optical sensitisers established by H. W. Vogel. It is conceivable that the regularly absorbing light-sensitive substance may be decompased by the action of light to form coloured substances also regularly absorbing and light-sensitive. I will designate as a colour receptive substance a black regularly absorbing light-sensitive substance whose products of decomposition consist only of monochromatic regularly absorbing light-sensitive substances of at least three radically different colours, and, besides these, of a white substance which, however, is the least readily formed. These colours must be radically different in order that by their mixture with one another and with white all compound colours may be possible. In distinction from these compound colours the unmixed colours will be called ground colours. The monochromatic substances reflect
only one colour well. They must absorb the others the more completely they differ from them. With these preliminaries it may be shown that a colour-receptive substance repro duces the colour of the illumination correctly. First, let the colour of the illumination agree with a ground colour. It will be absorbed by the black body and produces a decomposition substance which, by hypothesis, is regularly absorbing and light-sensitive. In this decom position different coloured substances are formed. Those not agreeing in colour with the incident light absorb it, since, by the hypothesis, they are monochromatic, and must absorb all illumination different from their colour. Since these are regularly absorbing light-sensitive substances, they are also decomposed by the light which they absorb. On the other hand, the substance of the same colour as the incident light is not decomposed, since it does not absorb. In the end, therefore, it alone can remain in company with the white substance. The amount of the latter is, by hypothesis, very slight, and its effect upon the colour is, therefore, noticeable only under strong illumination. Where the colour of the illumination differs from that of a ground colour, but is intermediate between two ground colours—as would, for example, be the case with green, and if yellow and blue were ground colours—the coloured substances would suffer least decomposition which reflect green best—that is, the yellow and blue. A green mixture would thus arise besides the small quantity of white. In white light all the colour substances would be decomposed, leaving white alone. In the absence of illumination the substance would remain black." Incited by Wiener's theorising, Vallot (Mon. d. la Phot., 1895, p• 318) used aniline purple (red), Victoria blue and turmeric on paper and exposed for three or four days to sunlight and obtained coloured results. The subject was followed up by Worel, who published (Auz. K.K. Ahad. Wissent. Wien, 1902) details of his process, though he had shown results at the end of 1901, and he used anethol as a sensitises. Neuhauss followed up the subject (Phot. Rund., Jan., 1gn2, and Eder's Jahrbuch, 1902-3-4), using oxidising substances, such as hydrogen peroxide and the persulphates, as sensitisers. Szczepanik (Phot. Korr., 1902), instead of mixing the dyes, coated them in three superimposed layers, and in 1906 a com mercial paper, " Uto," was placed on the market by Smith and Co., of Zurich. From a practical point of view all these preparations leave much to be desired, but such a paper with reasonable sensitiveness would be of great practical interest.