PhotoHistory

THE BRITISH JOURNAL OF PHOTOGRAPHY
No. 1392. VOL. XXXIV. JANUARY 7, 1887.

THE PROGRESS AND DEVELOPMENT OF ORTHOCHROMATIC PHOTOGRAPHY

By CHARLES EHRMANN

One of the greatest advantages in depicting objects by photographic methods is the truthfulness and accuracy with which they are rendered upon glass, paper, or other carriers of sensitive films. The remotest fiber and infinite structure of microscopic animals is reproduced by the camera with such exact sharpness, that student or investigator is enabled to project them in enormously enlarged proportions with equal exactitude. Architectural drawings and other designs are reproduced in correct angles, perfect copies of the original, and the human face is found upon the photographic plate not only as a true image but with all imperfections nature has given it. Every line, every elevation, is there equally sharp with hair or beard, and all is reproduced without mercy to the original.

Considering these and similar points, photographs are indeed correct representations. In one respect, however, photographic pictures are almost devoid of veracity, that is, in the reproduction of color values. When we examine photographic pictures, it is seen how very little truth there is actually in them in regard to color. Scarlet and yellow, which appear bright to the eye, are represented in a sombre, dark tone, and blue or violet, no matter how dark in the original, are, in the picture, quite light, at times even white.

Light, divided into its spectrum rays and then projected upon a sensitive photographic film, will not act in proportion to its color brightness, or as the eye is impressed, but in proportion to a certain force or power called actinism. The chief action will take place in the spectrum region from violet to blue; and those parts containing the bright colors, green, yellow, orange, and red, will scarcely be acted upon, showing that the visible effect on the eye is not at all a representation of the chemical or actinic force. The term photography is, therefore, not absolutely correct, for light in its entirety does not right, but a portion of it only, the chemically active or actinic rays.

With the rapid strides which photography has made in late years, a mode of operating has been discovered, which has not emanated from the brains of one single man, but which to accomplish all enthusiastic researchers in photo-chemistry and spectrum analysis have contributed. We have seen how the luminous light rays act upon photographic plates but to a limited degree, or not at all, and how others, appearing much darker, show excessive force. Pigment colors in action correspond very nearly to those of the spectrum, although the latter are preferable for experiments, or the determination of the action of a certain ray. The more refrangible rays, violet and blue, are actinic, that is, photographically active, while the luminous rays, red, yellow, &c., have but little photographic action. The end in view was to use these latter rays in photographing as well as the former.

The elder Dr. Draper experimented on the action of spectrum colors as early as 1850, and came to the conclusion that only those rays become active upon sensitive bodies which are absorbed by them. Extensive studies in spectrum analysis led to various experiments, and the propounding of various doctrines. Ducos du Huron and Cros interposed colored glass screens between objective and sensitive plate to intercept rays, and to control or to depress their force. A yellow screen depresses the action of blue, and red that of green.

The first yellow colored plates came from England, but they had no practical value, although they gave an impetus to more direct investigations and experiments on the absorptive power of colored films. If plates are colored with a yellow or orange dye, blue and violet rays will be intercepted, and the force of their action reduced. Green will act similarly on red rays, and red on green. The first attempts of scientific value were based directly upon the results of spectroscopic researches, and two colors, coralline and aurine, were employed to counteract the violent force of violet and blue. Not less objectionable were those of yellow and red.

At the occasion of the sun’s eclipse in 1873, a meeting occurred between a German investigator and Captain Waterhouse, of Calcutta, who was then induced to resume and continue his experiments, in consequence of which a newly discovered dye, the eosine, was proposed for coloring plates.

In the meantime American scientists had not been idle; the eminent Carey Lea made the subject a part of his studies; Frederick C. Ives employing the green chlorophyll for the depression of red, and Edward Bierstadt, in many directions experimented in orthochromacy.

In 1881, Clayton and Atout Tailfer, Frenchmen, patented and published a method (isochromatic they called it), with which the desired effects could be obtained to a very high degree. They added eosine to the emulsion of bromide of silver and gelatine, and gave a still greater susceptibility by adding small quantities of ammonia. The process was enthusiastically received, for with these eosine plates and the use of a yellow screen, blue and yellow were reproduced nearly correct, although the other rays did not respond so well. Highly interesting experiments with important results then followed rapidly, and some of the most renowned scientists resumed photo-spectrographic studies, and directed their attention to the action of the less refrangible rays, yellow, orange, and red.

The important labors of Victor Schumann, Dr. Eder, Charles Seolik, and Captain Abney led to the most marvelous results in orthochromatic photography, and among Americans we find Ives and Bierstadt again at the front, the former directing his attention to the action of chlorophyll in combination with eosine and other dyes.

Now we are able to copy the sun spectrum and nearly all pigment colors upon the sensitive plate. Of more than two hundred dyes experimented with, only a few have been retained for their efficient actions: eosines in various shades, blue and red chinoline, some of the anilines, and of vegetable substances, chlorophyll and turmeric. The mode of applying these colors to the sensitized film has been greatly simplified, and the photographer is now enabled to prepare a color-sensitive plate for immediate use.

Ordinary gelatine dry plates, preferably those which carry in the emulsion but a very small amount of iodide of silver, are subjected to an extremely diluted bath of ammonia to give them susceptibility for the dye, and are afterwards subjected to the action of erythrosine — a dye belonging to the eosine series and also in very diluted solution. The preparation of a plate requires not more than four or five minutes, and no other precautions are required than to work in nearly total darkness and to subject the plate to the coloring bath but for a limited time. Too much color will reduce the general sensitiveness. In contradiction to the term isochromatic, the method is now called orthochromatic, meaning to reproduce colors correctly as to their brightness, while isochromatic implies uniformity of colors. A better and more descriptive word would probably be the plain English color-sensitive.

The practical value which orthochromacy offers to scientific pursuits cannot be over-estimated, when we consider that objects formerly quite inaccessible to the power of a sensitive plate can be successfully photographed now. Prof. Pickering, of Boston, and the Brothers Henry, of Paris, have obtained pictures of the optically invisible nebulae of the Pleiades, and Prof. Huggins has made a photograph of the corona of the sun when not eclipsed, while Spitaler, of Vienna, has made pictures of the moon with a telescope corrected for optical rays only. Microscopic objects are in frequent cases copied with the aid of chinolines and erythrosine, and spectrum analysis finds in color-sensitive plates an indispensable aid. The professional photographer who reproduces with them oil paintings and other works of art, is enabled to make copies of fabrics or embroideries, no matter how greatly non-actinic colors may be prevailing. The retoucher’s work in portraiture will, in a measure, be dispensed with, for light yellow or brown freckles do not show on an orthochromatic negative, and light auburn or gold colored hair will photograph light, not black as heretofore.

With the extreme sensitiveness of erythrosine plates for yellow, orange, and red rays, artificial light has been pressed into the service of the photographer. Incandescent electric light, the gas flame, and even the ordinary petroleum lamp, give good light to photograph on color-sensitive plates.

While great credit is due to German scientists for having done the pioneering in spectrum photography and orthochromacy, Americans have not remained inactive, and they have contributed much to make the methods of photographing upon color-sensitive plates practical for the daily use of professional and amateur photographers.

Photographic Times.