THE CHEMICAL RADIATIONS. 



23 



the colors become the guides of the artist in conducting his processes. 

 When a person sits before the operator's camera, ready to be "taken," 

 the radiations which are reflected from his face into the instrument, 

 and collected to a focus by the lens, form three pictures, one behind 

 the otlier, the thermal, the luminous, and the cliemical image. The 

 luminous image is visible upon the ground-glass plate, giving all the 



R o Y G B I V 



Fig. 2. Intensities op the Forces op the Spectkum. 



colors of the object, but the chemical image is now blurred, and the 

 focus has to be readjusted so that the chemical picture will be clearly 

 and sharply defined ; but, as this image is invisible to the operator, 

 be has to make his readjustments by rule. As he cannot reproduce 

 the colors in the photograph, he has to substitute for them tints and 

 shades; but the chemical force is so unequal in the diiferent coloi'S 

 that the natural eflfects of gradation in tone and shade are not brought 

 out in the picture. This is one of the embarrassments of the process. 

 From the representation In Fig. 2, we should infer that blue colors 

 would act energetically upon the photographic plate and the yellow 

 and red feebly, or not at all, because the chemical rays abound in the 

 former and are absent in the latter. Of this false workintr of lio-hts 

 Prof. Vogel says : " Blue generally works clear, yellow and red work 

 like black. The yellow freckles appear, therefore, in a picture as black 

 spots, and a blue coat becomes perfectly white. Dark-blue flowers on 

 a light-yellow ground produce in photography light flowers on a dark 

 ground. Red and also fair golden hair become black. Even a very 

 slight yellow shade has an unfavorable eifect. A photograph from a 

 drawing is often blemished by little iron-mould specks in the paper, 

 invisible to the eye. These specks frequently appear as black points. 

 There are faces with little yellow specks that do not strike the eye, 

 but which come out very dark in photography. A few years ago a 

 lady was photographed in Berlin whose face had never ])resented 

 specks in photography. To the surprise of the photographer, on tak- 

 ing her poi-trait, specks appeared that were invisible in the original. 

 A day later the lady sickened of the small-pox, and the specks, at first 

 invisible to the eye, became then quite apparent. Photography in 

 this case had detected, before the human eye, the pock-marks, very 

 feebly tinged yellow." 



