ORTHOCIIROMATIC or ISOCIIROMATIC PHOTOGRAPHY.—Synonymous terms applied to a process of obtaining photographic images of objects in, or nearly in, their true respective tone values. Ordinary gelatine plates do not, as is well-known, represent objects in their true relative degrees of light and shade as apparent to the eye, for the reason that they are most sensitive to the blue and violet rays, less sensitive to the green, and very slightly sensitive to yellow, orange, and red, whereas the eye is most sensitive to yellow and least sensitive to blue and violet. Those rays which produce the greatest effect on the eye have therefore the least action on the photographic plate, and vice verso.
As an obvious: consequence of this defect, blue and violet objects appear much too bright in a photograph while green, yellow, and red appear too dark. Thus, if we were to make a photograph of a yellow cross on a blue ground it would appear in the photograph as a dark image on a lighte background, whereas if an artist were making a picture in black and white he would represent it the other way, that is to say, the yellow cross would appear lighter than the blue ground, but in the photographic image it would be darker.
Fig. 318 is a table of curves taken from Professor Bothamley's paper on this subject. The curves represent the intensity of action, they rise to represent a large effect, and fall to show a diminution. The vertical lines stand for the Fraunhofer lines, and are lettered accordingly. Number t curve shows the visibility of the spectrum ; number 2 the effect produced by exposure and development of an ordinary gelatine bromide plate ; and number 3 the result when it is prepared with a sensitizer composed of an ammoniacal solution of rose bengal.
The effect of this defect in photography will be at once apparent. Untrue images of col ored objects are obtained, the foliage in landscapes appears too dark, the blue sky, or water reflecting it too bright. The atmosphere intervening between the camera and objects afar off causes a bright blue haze, destroying all the detail in the distance. In portraiture the effect is
quite as destructive, the tints of the flesh appear too heavy, sun freckles or any other slightly yellowish mark, perhaps almost imperceptible to the eye, appear glaringly prominent as black spots or patches.
As far back as 1865, it was noticed that lenses which were of a slight yellowish tint, produced perhaps by long exposure to light, gave photographic representa tions of colored objects in better relation. This led to the discovery that the interposition of a yellow transpa rent screen between the object and the plate, either before or behind the lens, would, by cutting off the greater portion of the blue and violet rays, and giving the green, yellow, and orange rays more time for action, produce images with nearly the same luminosity that the colors in the objects appear to the eye.
In 1873 Vogel* found that by treating certain plates with certain coal-tar dyes it was pos sible to make them sensitive to the less refrangible rays, namely, the green, yellow, orange, and red. Later on Waterhouse recommended eosine for the purpose, and a large number of dyes were afterwards found out to be serviceable for this purpose by many able scientists, such as Vogel, Waterhouse, Tailfer, Eder, Bothamly, Wellington, Ives, Abney, Schumann, and others. Among these dyes may be mentioned the following: eosine, cyanine, rose bengal, erythrosine, chlorophyll, rosaniline, and azaline.
The names given to this process of photography are Orthochromatic from Greek orthos , right, true; and chroma—color, and Isochromatic from isos—equal, and chroma—color. Both these terms, it will be seen, are exceedingly unsuitable, and liable to mislead the student, as they should in reality be applied to photographs in true or natural colors. Other terms, such as ortho photic, orthoactinic, paractinic, etc., have been suggested, but the term orthochromatic will no doubt remain for some time to come.