COLOR PHOTOGRAPHY. 176 COLORS. eye will see the objeet in its natural colors, pro- vided the intensity of the deposit of sillier on the oriyinal three negatives is so adjusted for each •negative that the intensities of these red, green, and blue sensations are exactly such as to pro- duce the proper color-sensations. To secure this intensity on the photographic plates, three photo- graphs of the object nuist be taken, each through such a colored screen as will transmit enough light of all waVe-lengths to produce the desired result on the plate. Thus, one plate is exposed in a camera in front of which is a screen which alloxs to ])ass a great deal of red and small amounts of yellow and green ; the second is ex- posed with a screen which is transparent to the green and slightly to yellow and blue; the third is exposed ith a screen which is transparent to blue and slightly to green and violet. It is a <|uestion of the most careful experimenting to find what photographic plates should be used, and what colored screens give the proper inten- sity with them. There are three ]>rocesses of color photography based upon this general idea. In the Ives process, tlie three photographs of identical sizes are taken simultaneously on three plates, each through its proper 'taking' screen. From these three nega- tives, three positives are made by contact: and these positives, each with its proper 'viewing" screen of pure red, green, or blue, i)laced in a so-called 'kromskop' in such a manner that sun- light is reflected through them and their screens, and all three pictures are seen superimposed ap- parently on each other. In another process in- vented independently by Professor Joty. of Dub- lin, and Mr. McDonough, of Chicago, the three colored screens through which the photographs are taken are combined by having a series of lines of these three compound colors ruled very closely- together on a piece of glass, every fourth line having the same color. A single photograph is taken through this composite screen; a positive is taken of this, and a viewing screen, consisting of a series of lines, one a pure red, the next pure green, the next pure blue, with the same spacing as in the taking screen, is sujierimposed on the positive, so that the colored lines come in ex- actly the proper positions; and this compound plate is used as a transpai-ency by holding it up to the light, or by looking through it at a piece of white paper which is brightly illuminated. Quite a different process, although of the same generalprinciple, has been invented by Professor Wood, of .lohns 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 may lie 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 composite 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 ))roper optical means, the object will be seen in its natural coiors, 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 method 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' jjoints the string vibrates exactly as if it were an ordinarj' 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 .between the nodes there will be chemical action, which is thus confined to plane surfaces, parallel to each other and very close together, their dis- tance ajjart depending upon the wave-number of the light. If this photographic plate is now .suitably developed, the nodal planes will be dis- 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 be 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 fallsnpon the film in its original condi- 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 proc- 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 Icnown.) 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-ilcDonough, which Aill be found discussed under Ttieee-C'oi.or Proce.ss, For additional in- foiTiiation upon the subject of color photography, the reader may consult: Wood, Philosophical Magazine, vol, xlvii, (London, 1899) ; Joly, Na- ture, vol, liii, (London, 1895-96) ; Lippmann, Proc. Hoyal Society of London, vol, Ix, (London, 1896) : Wiener, Smithsonian Report (Washing- ton, 1896) : Bolas, Tallent, and Senior, A Handhook of Photography in Colors (New York, 1900), COLOR PRINTING. See Three-Color Pro- cess; LiTHOGRAPIIV. COLORS, IMiLTTAHY AXD Naval. The term applied to the national flag or ensign wherever
