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 other, the thermal, the luminous, and the chemical image. The luminous image is visible upon the ground-glass plate, giving all the
Fig. 2.—Intensities of the Forces of the Spectrum.
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 different colors that the natural effects 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 working of lights 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 effect. 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 taking her portrait, 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."
