The above process is also applicable to many of the commercial printing-out papers, both gelatine and collodion, if they are immersed in a solution of acidulated zinc chloride, are then well washed and treated with ct ro per cent. solution of hydroquinone.
Krone (Darstellung der Natiirlichen Farben) has confirmed Valenta's statements, and also Poitevin's and Becquerel's.
So far, merely the practical side of the question has been considered. Schule assumed that the dark silver subchloride was oxidised on exposure to yellow silver chloride, and this chemical view was supported by many chemists, though physi cists considered that the colours were due to thin films, as in Newton's rings, and this was confirmed by the very thin films obtained by Becquerel in the electrolytic chlorising of his plates. The true explanation was first pro pounded by Wilhelm Zenker in his Lehrbuch der Photochromie (1868), and this has been purposely disregarded till this particular stage, because it forms a fitting connection to the next process to be here considered, Zenker examines the various methods which, up to 1868, had been suggested, and in most cases gives his criticisms of the processes founded on actual experiments ; his book (reprinted, 1900), therefore, is extremely valuable. Zenker states : " When different coloured rays pene trate a light-sensitive substance with equal intensity, they must act on the same in abso lutely equal manner ; every ray, no matter what its vibration may be, must act on all the particles of the substance which lie in its path, set them in motion, and thus chemically alter them. Thus, there may be a general darkening, colora tion or bleaching ; these must be under one colour the same as under the other ; a difference of colour is not conceivable. The facts are, how ever, quite different when the incident rays meet again outgoing rays of the same phase, that is to say, if we consider that with all these photo graphic processes the incident rays are again reflected. This takes place most strongly with the daguerreotype, but also with sensitive sur faces lying on other supports, and one can convince oneself of the quantity of reflected light by merely looking at it. Now, if two waves of the same phase meet they give rise to the phenomenon which is called, in the case of water, ' stationary or standing waves.' " The formation of these waves is shown in diagram A, in which the incident rays are shown in continuous lines, and the returning rays in dotted lines. In 8 it is assumed that the ray is reflected from a substance with higher refrac tive index, and in F with a lower refractive index, than G, the sensitive surface. At the
points b b the reflected ray augments the inci dent, whilst at a a the pull on the ether particles is in contrary directions ; consequently there is no movement, and therefore no light. As a matter of fact, in standing waves, the ether par ticles rise and fall, but there is no propagation of light. Now, it is obvious that if G is a film of sensitive substance, the action of light will be to cause a deposit of silver at the internodal planes b b, as shown by the hatched lines, and this deposit is exactly half a wave-length of the incident light apart ; therefore, it can only reflect light of that colour which has a wave length of double the distance b b.
The truth of this theory was disputed by Schultz - Sellack (Annalen d. Phys. u. Chem., 1871, p. 449), following which further doubt was created by Carey Lea's production of the photo salts, as he was able to prepare the coloured substances, produced by the action of coloured light on silver chloride already exposed, by purely chemical means in the dark. Otto Wiener (Wiedeman's Annalen d. Phys. u. Chem., 189o, p. 203) was able to prove experimentally the formation of Zenker's standing waves, and the following is a brief summary of his work : With homogeneous illumination, there is formed in front of a reflecting surface a series of standing waves parallel to the reflecting plane. This is shown at B, in which R R is the reflecting sur face, and the planes of equal phase are shown in section, the nodal points in dotted lines, and the waves in maximum action in continuous lines. Now, if a light-sensitive film be coated on the glass, the metallic silver resulting from the light action must be apart, but as this distance, one half wave-length for red light, would be about •3 a, it is obvious that it would be invisible to the naked eye. Wiener, therefore, coated his sensitive film, a b, at an acute angle with the reflecting surface, and it will be at once seen that the distance of the silver lamina: is increased, for if 2 A – is the distance between two maximum wave planes, the separation, b, of the silver A laminm would be : b.-- sin a. Wiener used collodio-chloride of silver emulsion thinned down with about 15 to 20 parts of solvent, and thus obtained a colourless, transparent emulsion in which the silver laminae were distinctly visible. Later, he was able to confirm this with silvered glass plates fumed with iodine.