Though we cannot here attempt to describe every process of colour photography hitherto invented, mention must be made of the Lippmann process, which, though one of the least practical for everyday purposes, is undoubtedly the most interesting of all from a scientific point of view. Invented by Dr. Lippmann, of Paris, in 1891, it is independent of the three-colour theory, and dispenses with all light-filters except the usual yellow "compensator," used before or behind the lens to cut off the excess of violet rays.

The process is based upon the undulatory theory of light, and is itself one of the most interesting proofs of the truth of that theory. Briefly, the undulatory theory regards light as being the effect of wave-like movements of the all-pervading ether, in the same way as sound is the effect of undulatory movements of the air. As in a scale of music the different notes are due to differences in the length of the sound-waves, so in the spectrum the various colours are the result of differences in the length of the light-waves. The longest light-waves by which our eyes are affected produce the sensation of deep red; the shortest produce the sensation of violet. Between these are yellow, orange, green, and blue, all produced by waves of different lengths. In actual measurement these waves are inconceivably minute, ranging in length from about 1/35000 in. in the red to about 1/65000 in. in the violet, a wave-length being the measurement from wave-crest to wave-crest (D to D, Fig. 69).

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Fig. 69.

If a ray of light is reflected back upon itself, in such a way that the crests of the reflected waves are in opposition to the depressions of the advancing waves, as shown in the diagram (Fig. 69), where A is the advancing and B the reflected wave, the waves will neutralise each other at the points where they cross (c C C in the figure), producing points of darkness, while the light action will be increased at the points where the crest of one wave is opposite the depression of the other (d D in the figure). In other words, the light will be split up into layers, with layers of darkness between.

The Lippmann photograph is taken in such a way as to fulfil the conditions necessary to produce this effect. A specially prepared photographic plate is used, the sensitive coating being practically grainless and perfectly transparent, so that the light may pass through it without being "scattered." The plate is exposed in an ordinary camera, with its glass side towards the lens and its film side in contact with a reflecting surface of mercury. The coloured rays from the objects that are being photographed, passing through the plate to the mercury, are reflected back upon themselves, producing the effect above described. The result, on development of the plate, is that the silver is deposited in microscopic layers, whose distance apart varies according to the colour of the rays which fell upon that particular part of the plate. This is diagrammatically illustrated in Fig- 70, where red, green, and violet rays respectively are represented as entering the film and being reflected back upon themselves, the shaded portions showing the points of maximum light action, where the layers of deposit are produced on development. No attempt is made in the diagram to represent the actual number of layers produced. They are even more inconceivably minute than the lightwaves themselves, as they occur at every half wave-length. Thus, in a film only 1/500 in. in thickness, a red ray may be represented by as many as 140 layers and a violet ray by as many as 260.

Such a photograph, viewed by transmitted light, is perfectly devoid of colour. But when a black backing is applied to it, and it is viewed at a certain angle, a wonderful change takes place. The light falling upon and reflected from it is decomposed by the microscopic layers of silver deposit, each set of layers obstructing and suppressing all the light-waves, except those whose length fits its own structure. Consequently, the light rays reflected to the eye from each point of the photograph are of the same wavelength, or colour, as those which fell upon that point when the plate was exposed in the camera, the result being a complete colour picture of remarkable brilliance.

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Fig. 70.