RECENT ACHIEVEMENTS IN COLORPHOTOGRAPHY*

BY HERBERT H. JOHNSONEastman Kodak Co., Rochester, New York

Processes of color photography all date back to the classicexperiment of Clerk Maxwell before the Royal Institution,London, in May, 1861. On this occasion Maxwell demonstratedthat any shade of colored light could be produced by combiningvarious amounts of three primary colors, red, green, and blue-violet. He used three separate lanterns and placed colored solu-tions before the lens of each. Ferric sulfocyanide was used forthe red solution, cupric chloride for the green, and an ammonia-cal solution of copper sulfate for the blue. When the light fromall three lanterns was projected on the same spot on the screen,a white area appeared; when the red and green beams weresuperimposed, a yellow spot was obtained; with red and blue,a magenta spot, and with green and blue, a blue-green spot.This is called the additive method of color synthesis, and is thebasis of the so-called additive methods of color photography.

Eight years after Maxwell’s demonstration a small Frenchbooklet was published in Paris written by Louis Ducos duHauron. This contained a description of almost all the basicprinciples of color processes which have subsequently beenworked out. Credit should therefore be given this French in-ventor for his foresight. In the intervening years until his deathin 1920, he continued to experiment and work out further de-tails of color processes, but was unable to realize much com-mercial success from any one of these methods.Of the many references published on color photography,

Wall’s ^History of Three Color Photography," which was pub-lished in 1925 is perhaps the most valuable, since it representsa compendium of information on all phases of the subject.

Color processes are generally divided into two types, the addi-tive and the subtractive. With the additive process the actualred, green, and blue colors are either separately visible in thefilm itself or are formed by an optical system as with Kodacolor.These primary colors are projected onto the screen for viewingin such a way that they are added together by the eye. Additive

* Lecture delivered before the General Science Section of the Central Association Meeting in St.Louis, Nov. 27, 1936.

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methods include Kodacolor, color screen plates such as theAutochrome, Finlay plate, Dufaycolor and others.Any possible color on the screen can be formed by additive

combinations of the primary colors, red, green and blue.Likewise they can be formed by proper combinations of the

colors which are complementary to the additive primaries. Thecomplement of red is blue-green, of green is magenta, and ofblue is yellow. These complements are sometimes referred toas the minus colors thus:

Primary Color Complementary Color

Red Blue-green called minus redGreen Magenta called minus greenBlue Yellow called minus blue

Subtractive processes form their colors by combining thethree complementary or minus colors. The minus or subtract!vecolors simply absorb the corresponding primary color from theprojection beam of light.

If we put a blue-green, which is to say, minus red filter ordye deposit, into a white light beam, the red will be absorbedand the beam will appear blue-green.

If, now, we add a magenta (minus green) filter to the blue-green filter mentioned above, both the red and the green lightwill be subtracted from the white light beam. The only lightleft to go on through is blue. Similarly any color may be formedby suitable superposition of layers of the subtractive colors invarious densities.The new Kodachrome process is a subtractive process. The

colors of the original subject are visible in the finished positive.In all subtractive color prints the three separate subtractivecolors are superimposed. Usually three separate negatives aremade through the three primary filters, and prints are madefrom these in dyes complementary to the taking colors, andthen superimposed on one support.The important point that makes Kodachrome different from

other color processes is the fact that instead of making thethree negatives on threee separate films, they are separated inthe depth of one film. Kodachrome film is coated no less thanfive times! Nearest the base an emulsion is coated which is ex-tremely red-sensitive. On this is a very thin coating of gelatine.On top of this is coated a second layer of emulsion which is

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strongly green-sensitive; over this is coated a second coatingof gelatine containing a blue absorbing eye, and finally, on thetop, is coated an emulsion which is blue-sensitive.Kodachrome film resembles a three-layer cake with the frost-

ing left off the top layer. The three layers of emulsion are rep-resented by the three layers of cake. The gelatine coatings be-tween the bottom and middle and between the middle and topare represented by the frosting. All five of these coatings com-bined are no thicker than an ordinary black and white film.The exposed film is developed by the reversal process used

for the past twelve years for black and white 16 mm. photog-raphy. In developing Kodachrome, an image corresponding tothe blue component of the subject is formed in the top layer,that corresponding to the green in the middle layer, and thatcorresponding to the red in the bottom layer. These imagesare then converted into dyes the colors of which are comple-mentary to the colors by which they were made: top, yellow;middle magenta; bottom, blue-green. The finished color imagesare perfectly registered one over another.There is no mechanical registration to give blurred pictures;

there are no lines or screen pattern on the film and no graininess.The wonderful life-like naturalness of Kodachrome picturesmakes one feel as though he were looking out through an openwindow at real life.

I am sure after you see some of the pictures which we arenow going to project, that you will agree that Kodachrome isthe greatest achievement in color photography thus far. It isimpossible to predict the future of this process, but the possi-bilities appear to be unlimited.

WORLDS BIGGEST OYSTER-SHELLBiggest oyster-shell in the world is the distinction claimed for an enor-

mous fossil, four feet long and three feet wide, dug up by geologists of theNational Park Service in the proposed Big Bend National Park area, inwestern Texas. Dr. Charles N. Gould, who has been working in the richfossil deposits of the region, thinks there may even be bigger shells waitingto be excavated.Another "biggest" fossil discovered in the Big Bend country is a ten-

foot petrified tree. No other tree of that size, living or fossil, has ever beenfound in Texas. The region is also rich in fossils of dinosaurs and extinctelephants.