In 1895, Wiener (Annalen d. Phys. u. Chem., 1895, p. 225) examined the question of the See beck and Poitevin processes on paper, to deter mine whether the colours were apparent or body colours—that is to say, whether they were pro duced by interference or absorption. To deter mine this question, he used a right-angle prism of highly refractive glass, =1•75 for D, with its hypotenuse on the colour pictures, the intervening air space being filled with benzole so as to prevent total reflection, and thus securing for light rays entering normal to the side surfaces an angle of incidence of 45°, so that the ray entering the silver chloride must form a consider able angle with the normal to the surface. The difference in path of the interfering light waves will, in comparison with vertical incidence, be greatly changed, and according as the colours are thus altered or not they are interference or body colours. The prism was so placed upon one half of the photographed spectrum that the line between the hypotenuse and the side face (diagram C) cut similar colour lines at right angles. The eye of the observer was placed in the prolongation of the same surface I (the arrow indicates the line of vision) so that a line, s, drawn before the experiment, in the direction of a single colour—as, for example, the yellow— appeared straight when viewed through the air and prism. D shows another arrangement, in which the spectrum reproduction was cut in half perpendicularly to the reference mark after draw ing the latter in the yellow. One of the parts was placed upon the side of an auxiliary prism, fastened to a level glass plate, upon which the other half of the sheet was so placed that the marks came together. Finally, prism I, with the high refractive index, was set upon the second half, benzene poured between, and the eye placed in line with the reference mark and with the surface of the principal prism. Wiener thus sums up his conclusions drawn from the above experiments : In the Seebeck and Poitevin pro cesses there is no change of colour under the above conditions, and they are, therefore, body colours ; this is also confirmed by the fact that these pictures show the same colours by trans mitted as by reflected light. The colours of the Becquerel pictures, produced on an underlying silver mirror, were chiefly produced by inter ference. Carey Lea and Krone proved that the substances present in the Seebeck and Poitevin processes were capable of yielding compounds which embrace almost all the spectral colours, if not all their tones, and the reason why they agreed in hue with the illumination-producing decomposition is that of all the coloured sub stances capable of being produced, only those will be stable which agree in colour most nearly with the incident light, since these will best reflect and least absorb it, and can, therefore, be least changed. Decomposition products of other colours, on the other hand, absorb this light, and will be again decomposed. Proof of this was found when a spectrum was thrown at right angles on a colour photograph of the spectrum. It was then found that a correctly reproducible illuminating colour was capable of decomposing all colours differing from it, but similar colours remained unchanged. It is, therefore, fundamentally possible that coloured illumination shall, in suitable substances, produce similar body colours, and such substances Wiener termed " colour receptive." In 1891, Prof. Gabriel Lippmann, lecturing at the Academie des Sciences (Comptes Rendus, 1891, p. 274), explained at length the theory of inter ference, and exhibited actual photographs in colours produced by his method, which is essen tially that of Wiener, and theoretically described by Zenker. He used a transparent gelatine emulsion of silver bromide with the sensitive film in contact with a film of mercury and the glass of the plate presented to the lens. From what has already been said, it will at once be seen that here was the ideal method of recording, if possible, the formation of standing waves, and if formed they would naturally reflect the colours of the incident light.
Lippman_n's own description (Phot. Journ., 1897) is by far the easiest to grasp, and he says : " Now, how is it that we see the colour ? The photographic operations are the same as in ordinary photography, the result of the opera tions is the same, a similar deposit of reduced silver is obtained, and the materials of which the image is composed are the same as those in a colourless plate. The difference is that the plate has acquired such a structure that it decomposes the light by which it is illumined, and sends back to the eye of the observer elements of white light, which together make the natural colours of the object. In the same
manner the colourless drops form the rainbow. A soap bubble appears coloured, although consisting of a colourless solution, and mother of pearl appears coloured although made of colourless carbonate of lime. It is a phenomenon of interference due to the structure which the deposit has acquired ; if you were to use a plate without a mirror you would get an ordinary negative, but the presence of the mirror changes everything, and this is how it is done : You know that light is made up of vibrations, just as sound is ; these vibrations give rise to light waves that rush through the ether and the plate with a velocity of 300,00o kilometres per second ; therefore, they impress the plate more or less strongly, and thus leave a design of different intensities of the image, but as they rush through the plate they leave no record of their form. And this is what I mean by their own form : Each ray of light of a certain colour has a certain structure ; it is made up of waves which have a certain wave-length ; you know a wave-length is the distance between the crests of two succeeding waves ; red has a com paratively great wave-length, blue has a much smaller one, and the intermediate colours have each a distinct and intermediate length of wave. If you put no mirror, each train of waves rushes through the plate and wipes off every record of its own form by reason of its velocity ; you cannot expect a thing which moves with a velocity of 300,00o kilometres in a second to give a photograph of itself. If you put a mercury mirror behind the plate, then the following phenomena occur : The light is reflected back on itself ; the light rushes in with a velocity of light and rushes out again with the same velocity ; the entering and issuing rays interfere, and the effect of the inter ference is that vibration takes place, but the effects of propagation are stopped, and instead of having propagated waves we get stationary waves—that is, the waves now rise and fall, each in its own place ; they pose, therefore, in the interior of the film and impress their form upon it, the largest movement giving the strongest impression, and where the move ment is naught the impression is naught. So that you have the form of the vibration impressed in the interior of the film by the photographic process, and the photographic film has really now acquired the structure of the incident rays, because they have become stationary, and impressed their form upon it. The result is, that if you look through the film you see nothing special ; it looks like an ordinary negative ; but if you look at it by reflection, then you see it coloured. And this for the following reason : Suppose at one place the plate has been impressed by red light, the red light has impressed its structure on that part of the plate, and that part of the plate is now able to reflect back to our eyes only the red part of the white rays—only the red element which is a com ponent part of white light, and similarly with every part of the spectrum ; it is a mere me chanical adaptation of the form of the deposit to the form of the light vibrations." Now, if the capability of one of the Lippmann heliochromes to reflect the colour of the original incident ray back to our eyes is dependent on the distance of the laminae of silver one from the other, it is obvious that if we could increase this distance the colours should change. This is precisely what takes place. It is only neces sary to expose a heliochrome to steam or aqueous vapour, which is absorbed by the gelatine ; this swells, and the distance between the laminae is increased. Now, assuming that this distance of separation for the extreme violet be half one wave-length, 4,000 ten millionths of a millimetre, that is, 8,000, and we steam it so as to increase the distance by one-fourth, then our 8,00o becomes io,000, and one-half of this would be 5,000, which is the wave-length of the bright blue ; therefore, this would be the colour reflected to the eye. This change of colour with steaming or absorption of aqueous vapour can be proved with any heliochrome. It is obvious that there might well be heliochromic effects produced on daguerreotypes, with which a polished silver plate with a sensitive film is used, and such coloured results have actually been observed (J. Nicephore Niepce).