Quite a different process, although of the same general' principle, has been invented by Professor Wood, of Johns Hopkins University. It depends for the production of the pure colors, red. green, and blue. upon the phenomena of diffraction gratings (q.v.), by means of which white light tuns be dispersed into pure colors. It is neces sary to have three gratings ruled on glass, with grating spaces such that one gives the same deviation for the green as the other two do for the red and blue respectively. Three negatives are taken of the natural object, each through its own compound color screen; on these the three glass gratings are superimposed each on the proper one positives are then taken through these compound negatives: the three images being superimposed by suitable lenses, thus forming a eornposite positive of the natural object over laid with parallel lines suitably spaced and placed. By viewing a source of white light through this plate and using proper optical means, the object will be seen in its natural emors, the dispersed colors of the three gratings serving in place of the viewing screens of the former processes.
An entirely different physical principle is made use of in the Lippmann of color photog raphy; it depends upon the fact that the colors seen by the eye are caused by ether-waves of different wave-number; and so, under proper precautions, it is possible to have 'stationary vi brations,' so called, produced. If one vibrates rapidly the end of a long rope, the other end of which is fastened to a wall, waves are sent along the rope; reflected waves are produced; and, as the direct and reflected waves are thus super imposed, there are certain points, regularly spaced, where the two waves neutralize each other's action, while in between these 'nodal' points the string vibrates exactly as if it were an ordinary string stretched between two fixed pegs. This is called a stationary vibration; and the distance between two nodal points equals half the wave-length of the train of waves which is the original cause of the vibration. The same phenomenon may be produced by ether-waves if allowed to fall upon a mirror. In Lippmann's
process a photographic plate of particularly fine grain is placed so as to form one side of a bath containing mercury, the film side being away from the mercury. If light of a definite color falls upon the photographic plate, the waves enter the film. reach the mercury. are reflected, and form stationary vibrations. In the nodes there will he chemical action, . .
which is thus confined to plane surfaces, parallel to each other and very close together, their tance apart depending upon the wave-number of the light. If this photographic plate is now suitably developed. the nodal planes will be solved out largely, thus forming of the film a pile of parallel plates at minute intervals. If such a pile of plates is viewed with white light, it will appear to lie of the same color as that of the light which produces the chemical action, owing to the phenomena of interference (q.v.). Similarly, if the colored light from any natural object falls upon the film in its original tion, each color will produce its own stationary vibration and its own set of parallel planes, where there is chemical action; and so, when developed and viewed in white light, the image will have the proper colors of the object itself. (The above explanation of the Lippmann ess is not complete: it offers but a rough idea as to what takes place. In fact, a satisfactory explanation of all the phenomena is not known.) To print in the natural colors the photograph of any colored object is perfectly possible by a simple modification of the method of Ives or of Joly-McDonough. which will be found discussed under THREE-COLOR PROCESS. For additional in formation upon the subject of color photography. the reader may consult: Wood, Philosophical Magazine, vol. xl•ii. (London, 1 8 99) ; Joly, ture, voI. liii. (London, 1895-96) ; Lippmann, Proc. Royal Society of London. vol. lx. (London, 1896) ; Wiener. 8azithsonian Report (Washing ton, 1896) : Bolas, Tallent, and Senior, A Handbook of Photography in Colors (New York, 1900).