COLOUR SENSITISING It was very early recognised that certain colours acted more strongly on the photographic plate than others, and that the former were nearly all those that reflected the violet and blue spectral rays, which therefore were called the " chemically active " or " actinic," whilst green, yellow, orange, and red hardly produced any action at all. This view prevailed for many years, though Herschel pointed out in 1842 that it did not apply generally, as violet petals faded most quickly in green light and the other blue flowers faded most in yellow light. Draper enunciated the law that only those rays acted chemically on a substance which were absorbed by it.
In 1873 H. W. Vogel was examining various silver salts in the spectrograph and found that some English collodion plates, which had been stained with a yellow dye to prevent halation, were distinctly sensitive to green. Led by this fact, Vogel examined the absorptions of various dyes and then added them to collodion emulsion, and found that these also sensitised for the colours they absorbed. This principle was con firmed by others, and soon after was successfully applied to commercial gelatine plates by Attout Tailfer, of Paris.
The action of a very large number of dyes has been examined by various authorities, Eder, Valenta, Von Hiibl, Eberhard, Hinterberger, Ruh, etc., and the first-named, who has paid special attention to this subject, has formulated the following important conclusions : (1) The absorption spectrum of neither an alcoholic nor of an aqueous solution of the dye coincides with the position of maximum light action on the dyed gelatino-bromide of silver. (2) The maxi mum of sensitiveness of the dyed silver bromide lies nearer the red than does the absorption maximum of any solution. (3) The position of the maximum of absorption of the dye in gelatine and that of the maximum of sensitiveness of the dyed silver bromide differ generally by about thirty wave lengths; that is to say, those rays of light that are most active photographically on the dyed silver bromide possess a mean greater wave length of about thirty wave lengths than those that are absorbed by the dyed gelatine (without the silver bromide). (4) The absorp
tion maximum of silver bromide dyed with eosine coincides exactly with the maximum of light sensitiveness on silver bromide dyed with eosine ; that is to say, those light rays which are absorbed by eosine-dyed silver bromide have the same wave length as those for which the dyed silver bromide shows the increased sensitiveness. (5) The dyes must stain the silver bromide grain ; the dyes that act vigorously are all " substan tive " dyes. (6) They must show in the dry state—on dyed gelatine, or more correctly, on dyed silver bromide—even in considerable dilution, an intense absorption band if they are to produce an intense action on the silver bromide. A narrow absorption band gives a narrow sensitising band.
Although it has been established that a colour sensitiser must dye the silver bromide grain, yet all dyes that stain the silver bromide are not sensitisers. Neither fluorescence nor fugitive ness to light plays any important part, as pare erythrosine is not fluorescent and yet is a power ful sensitiser, and is fairly stable to light, whilst cyanine is very unstable and is a good sensitiser. Apparently, there is no connection between the chemical constitution of a dye and its sensitising powers, though Lumiere and Seyewetz have concluded that the sensitising action is con nected in some way with the chromophoric group of elements. Joly has pointed out that all the sensitisers are photo-electric, and assumes that electrons are set free from the dye which act on the silver halides.
Colour sensitive plates may be divided into practically two main classes, the commercial isochromatic or orthochromatic plate and the panchromatic plate. The former are usually prepared by adding erythrosine to the emulsion, either at the time of mixing or just before coat ing, and are sensitive mainly to yellow-green and yellow, there being a characteristic gap or lack of sensitiveness in the blue-green. The pan chromatic plates are nearly all made by bathing the finished and dried plates in a solution of the erythrosine.