COLOE PHOTOGRAPHY. 195 



can be secured. It appears not impossible that the color substances 

 produced should by chemical action be converted into similarly colored 

 stable substances, or by a suitable addition be protected from decompo- 

 sition. A case of the latter kind is mentioned by Otto 1ST. Witt in a 

 very excellent memoir. 1 



There are, he says, fading dyes — that is, light-sensitive substances — 

 in threads which are made lasting by saturating the thread with copper 

 salts. According to Witt's hypothesis these copper salts have no influ- 

 ence on the dyes, but on account of their easier decomposition absorb 

 the light energy and make it harmless to the colors. 



It is also conceivable that the photographic film might be made light 

 sensitive by the addition of other substances and again become insen- 

 sitive after their withdrawal. It may be asked what object there is 

 in seeking for new processes when excellent ones are already at hand. 

 Experience, however, shows that where there are different solutions of 

 a technical problem it is seldom that any one supersedes all the others. 

 Each retains that province for which it seems best adapted. And if 

 the body-color processes are at present the most incomplete, the future 

 investigators can not now be told within what limits these imperfec- 

 tions shall remain any more than future generations can be informed 

 within what bounds their knowledge will be included, as is exampled 

 by those who in times past have thought to determine such limits. 



XV. — Mechanical Color Adaptation in Nature. 



A far-reaching influence has been already ascribed to light in the 

 production of the colors of nature, 2 not only in plants, whose green is 

 attributed to the action of light by all, but also in animals. Such a 

 direct influence is, however, generally denied, or at least only recog- 

 nized in a restricted way, most scientists, with Darwin, attributing the 

 coloring of animals to the action of natural and generic selection. 



iOttoN. Witt, "Ueber Farben und Fiirben. Eine Studie iiber Energiever wand- 

 lung." Vortr. geh. bei Gelegenheit des VI. deutscb. Fiirbertages. Prometheus, pp. 

 625, 641. 1894. He remarks very significantly tbat tbeory and practice have ceased 

 to be strangers to eacb other, since each theoretical advance is followed by one in 

 practice. This is certainly true of the development of color photography, and it is 

 to be hoped in the case at hand. I can not, however, leave unchallenged one state- 

 ment of the author, namely, his assumption that the chemical action of the long- 

 wave lengths is possible only by their conversion, after absorption, into short waves. 

 One might with equal justice say that the heating effects of short wave-length 

 radiations are to bo explained by their preliminary conversion into long. The 

 nature of tbe action of light is, however, determined, not by the wave lengths, but 

 by the characteristics of tbe receiving substance. My experiments with stationary 

 light waves show that the chemical action is caused by indwelling electric forces, 

 and these are, of course, independent of the wave length. They can, according to 

 the nature of the receptive substance, produce either decomposition or heating, just 

 as the electrical energy of a constant current may produce decomposition of electro- 

 lytes or heat in a metallic conductor. 



3 See Karl Semper, Die natiirlichen Existenzbedingungen der Thiere, Leipzig, F. A. 

 Brockhaus, 1880, page 107. 



