ORTHOCHROMATIC OR ISOCHROMATIC PLATES.—Photographic plates prepared with a dye, either added to the emulsion previous to coating the glass, or applied in solution to the ordinary gelatino-bromide dry plate. By this means better and truer color gradation is obtained.
These plates may be obtained commercially, and are equally adapted for landscapes and for general work in the studio. They will keep equally as well as ordinary gelatine plates, they are quite easy to work, and owing to their color sensitiveness, give better gradation and a more truthful representation of natural objects in the light and shade of the photograph. These plates are made in three degrees of sensitiveness, i.e., "medium," "slow," and "instantaneous." The following instructions for use are given: These plates are more sensitive than ordinary plates to the usual red or yellow light of the dark room; therefore great care must be taken to use only a dull light of a deep ruby color, and to shield the plate as much as possible during the manipulations. It will be found a good plan to cover the dish until the development is nearly completed. An amount of light which would be perfectly safe with ordinary gelatine plates, is quite sufficient with these color-sensitive films to cause fog and spoil the negative.
Exposure.—These plates are extremely sensitive to ordinary daylight, and require very short exposure in the camera. If artificial light be used, such as gaslight, their relative sensitive ness will be found many times greater than the most rapid gelatine plates of the ordinary kind.
For photographing colored objects, such as oil paintings by daylight, it is sometimes advisable, in order to obtain to their fullest extent the isochromatic qualities of the plates, to use a color screen of yellow glass behind the lens, and proportionately increase the exposure. For ordinary purposes no screen is required. (See Orthochromatic Screen.) Development.—Any good formula for developer may be used. For pyro-development the following is recommended : When required to develop a plate, mix equal parts of these two solutions just before use ; place the exposed plate face up in a shallow dish or tray, and pour the mixture steadily over the plate, avoiding air bubbles ; rock the dish gently, taking care to keep the plate well covered with the solutions. Do hot hurry the development, but allow the plate to remain in the solution after
all the details are visible, until the required density is obtained. With these plates and the above developer there is no danger of fog, except from light or over-exposure.
Immerse the negative in the above, which should be freshly mixed.
Clearing fixing, rinse under the tap, then flood the plate or immerse for one minute in the following solution : Alum i ounce Sulphuric acid ounce Sulphate of iron 3 ounces Water 20 ounces Wash well and dry as usual.
Negatives on these plates develop without difficulty to full printing density. If found too intense they may be further reduced by allowing them to remain a longer time in the clearing solution.
Considerable advantage is to be obtained by using these plates both for portrait and land scape photography. In the latter the improvement is at once seen, not only in the corrected rendering of the detail in the foliage and foreground, but also in the general effect of correct light and shade with an atmosphere that would be ruinous in ordinary photography, wholly obliterating the distant background.
In a recent issue of Les Annales Photographiques, M. L. Tranchant gives some important additions to the subject of orthochromatism, and we translate extracts from his article to form a valuable addition to the series of articles upon the subject by Mr. P. C. Duchochois, which we recently published,* and who may fairly be said to be the propagator in this country of the ortho chromatic process. For many years past he has published numerous articles pointing out the advantages of orthochromatic plates over the ordinary ones for all classes of work, whether studio, landscape, or instantaneous.