532 THE POPULAR SCIENCE MONTHLY. 



removed from the hypo, was washed and dried as usual ; but, when I 

 examined it by reflected light, it turned out that the green tree was 

 colored dark green just as nicely as if it had been a camera image, and 

 the red house was not a bit behindhand in truth and delicacy of hue. 

 A photograph in colors, sure enough ! But an examination with a lens, 

 and a little turning and twisting of the plate, caused the illusion to van- 

 ish : the colors were those of " thin plates," soap-bubble colors, caused 

 by the interference of light. "Wet collodion often shows them in patches 

 when it is somewhat rotten, and this sample was very rotten. The 

 interference effect had nothing to do with the color of the light, but 

 was controlled by its intensity. Once I had a chance to examine some 

 photographs in color of gayly dressed dolls made by Kiepce de Saint- 

 Victor, and it seemed to me that the pale colors they presented were 

 produced by a species of interference, acting by means of the presence 

 of more or less finely divided particles. The details of my examina- 

 tion I do not recollect, but merely the conclusion that the appearances 

 presented M'ere due to causes analogous to those that were effective in 

 the case of my glass negative. Photographs in color, such as they 

 are, can be obtained with suflicient patience ; but, in order to give this 

 fact the slightest value, it is necessary to prove that a corresponding 

 amount of patience would not be rewarded by the production of col- 

 ored photographs of objects which were gray, light gray, dark gray, 

 etc. When we think we have made a discovery, our first duty is to 

 destroy it mercilessly if possible, and the reproduction of the same 

 effects with white or gray objects is the proper mode of administering 

 justice in this case. It is barely possible that some one may ask why 

 a process that renders the colors correctly is a failure merely because 

 corresponding colors can be obtained when the natural objects are 

 tinted gray. The question answers itself ; white and gray objects 

 will be colored in the photograph, and, worse than that, the same color 

 in the natural object will vary in the photograph with its brightness 

 or luminosity. 



Let us now examine this subject from a theoretical point of view, 

 and ask ourselves why we should hope that photographs in color could 

 ever be produced. We see the rich red rays of the spectrum falling 

 on the plate, and we imagine that a substance which is sensitive to 

 light will somehow be acted on by them, and arrange itself so that 

 ever afterward it will better be able to reflect red light than any other 

 kind of light. Why ? Why should a substance that has been acted 

 on by long waves be better able to reflect long waves than those that 

 are shorter ? Why should a sea-beach that has been acted on by long 

 waves be on that account better able to reflect and redirect to the 

 ocean long waves rather than mere ripples ? The waves of light pro- 

 duce in sensitive substances chemical changes ; new compounds are 

 formed ; why should the long waves of red light produce compounds 

 that are especially capable of reflecting long waves, or red light ? 



