COLOR PHOTOGRAPHY. 189 



process. Sufficiently overexposed portions of the spectrum were white. 

 Beequerel ' himself says of his process that the differences of color dis- 

 appear with prolonged exposure. 



I have, therefore, exposed a Beequerel plate twenty hours, and a sec- 

 ond thirty hours, to illumination by the spectrum. The prism experi- 

 ment then gave, with the first, a slight color displacement; but with the 

 second the displacement was hardly noticeable. At the same time the 

 colors were very indistinct under the prism. 



A more thorough demonstration of body colors was to be expected 

 in examining the color-bearing film by transmitted light. The film 

 was for this 'purpose removed from the silver backing (see p. 184). 

 There then appeared in reality, by transmitted light, red and a trace of 

 blue in the proper places, the latter, however, being in the first plate 

 more of a grayish blue, and in the second of a violet-blue tone. 



It was, however, to be expected that interference colors would exert 

 a disturbing action. Hence, the side which had been next the silver, 

 and which showed brilliant interference colors by reflected light, was 

 rubbed with a leather pad till these colors were fainter. The red, in 

 particular, was now transmitted much more strongly, but it was no 

 more a spectrum red than in Seebeck's process. The same was to be 

 observed in a film exposed only three quarters of an hour, but less 

 distinctly. 



These experiments show, therefore, that in Becquerel's plates, also, 

 body colors are produced and cooperate to a greater extent the longer 

 the exposure is continued. 



XII. — The Theoretical Basis of a Method of Color Photog- 

 raphy with Body Colors. 



In order that a substance sensitive to light can be chemically changed 

 by the action of any kind of light it must absorb it. The converse 

 proposition is not general. The absorbed light can, for example, be 

 exclusively transformed into heat. A distinction is therefore made 

 between thermal and chemical absorption of light. 



For the sake of simplicity of expression I shall designate as a regu- 

 larly absorbing light-sensitive substance one which is sensitive to all 

 colors which it absorbs, and is affected by each color in proportion to 

 the capacity for absorption. That there are such substances, at least 

 to a considerable degree of approximation, is known. Upon their exist- 

 ence is based the important law of optical sensitizers established by 

 H. W. Vogel. 2 



1 Beequerel. La Lumiere 2, p. 222, 1868. 



2 The maximum of sensitiveness is, with respect to the absorption maximum, thus 

 far continually found to he displaced toward the less refrangible end of the spectrum. 

 The displacement of these maxima on the same plates has been investigated for a 

 great number of sensitizers by J. J. Acworth (Annal. der Phys. und Cliem., 42, p. 

 371, 1891). He finds not only great but also very small displacements. It is, then, 

 not impossible that there may be color substances in which the displacement is not 

 noticeable for the purpose under consideration. 



