EFFECTS OF DYES ON SENSITIVE FILMS.
In Dr. H. W. Vogel, of Berlin, found that when films were treated with certain aniline and other dyes, the plate showed an increased sensitiveness to those portions of the spectrum which the dye was capable of absorbing. Dyes which acted in this manner were called, by Vogel, "optical sensitisers " ; and, in his opinion, they might act in both an optical and a chemical direction. The propriety of the term " optical sensitiser " has been warmly contested ; the general opinion being that it is a misnomer, the action of the dyes being more of a, selective and chemical nature. Abney, in lti75, gave what is probably the true explanation, by demonstrating that certain dyes combined chemically with silver, forming coloured organic silver salts, sensitive to light. In confirmation of this, Dr. _Amory succeeded in obtaining a photograph of the spectrum by means of a sensitive compound of silver and cosine. Major J. had also been en gaged in similar experiments, and proved that the various rays of the spectrum took effect only when absorbed by the com pound. Another valuable and suggestive discovery by Abney was that, in many cases, the dyes themselves were reduced by the action of light, thus forming, as it were, a nucleus for the after deposition of the silver during development. Abney's theories have not gained universal assent.
Eder and Vogel held the view that the energy absorbed by the dyed silver salt is partially employed in effecting the chemical decomposition of the latter. In Lion of chlorine, and the consequent de position of metallic silver. It is necessary, however, that sonic body shall be present which is capable of absorbing the chlorine —or, at any rate, that the latter should be free to escape. A glass tube (Fig. 551) of dried silver chloride, from which the air has been expelled previous to its being hermetically sealed, will not discolour in the light in the slightest degree. If, how ever, a bent tube (Fig. 555), containing silver chloride at one end and a drop of mercury at the other, is scaled up in a similar fashion, the result will be different. The mercury gives off a certain 1895, Mr. C. H. Bothamley, in a paper read before the British Association, ably summed up the matter as follows :—" The balance of evidence is greatly in favour of the view that the dye absorbs the par ticular groups of rays, and, in some way which is not at all cleat, hands on the energy to the silver bromide with which it is intimately associated, and which is thereby decomposed." This part of the subject will be treated with further de tail in the section on " Orthochromatie Photography."