DYES AS COLOUR SENSITISERS The peculiar property possessed by certain dyes of conferring upon the silver halides in creased colour sensitiveness was discovered by Vogel in 1873, has had a most important influ ence on the advancement of photography in almost every branch, and since his time the number of dyes which act as sensitisers has been and is being continually increased, though the practically valuable ones are but few. It was but natural to expect that an examination of the chemical constitution of the various sensitisers would show some common property or grouping of elements to which might be ascribed the sensitising power, but up to the present no definite conclusion can be come to. It has been further suggested that the sensitive ness of the sensitisers themselves to light was the cause of their action, but some of the most fugitive dyes are not sensitisers, whilst others which are very stable are. Then, again, it has been pointed out that many of the dyes are photo-electric, and that here electrons may be set free which act on the silver halide, causing increased ionisation of that part of the silver halide which forms the latent image. The sub ject is so complicated, and the mass of material available for examination is so meagre, that the resulting definite conclusions are disappoint ingly small.
There are certain generally accepted facts as to sensitisers, and these have been most con cisely summarised by Eder as follows :—(i) The absorption spectrum of neither the alcoholic nor of the aqueous solution of the dye nor of the dyed gelatine agrees with the position of maximum action on the sensitive emulsion.
(2) The position of maximum action of the dyed silver bromide always lies slightly nearer the red than the absorption maximum of any solu tion of the dye. (3) The position of maximum of absorption of the dye in gelatine and the maximum sensitising action generally differ by about 3o p. p. or wave-lengths. (4) The absorp tion spectrum of a dyed silver halide coincides with the maximum sensitising action. (5) The dye must stain the silver halide itself to be a sensitiser, but all dyes that thus stain are not sensitisers. (6) Fluorescence, or fugitiveness to light, of the dye appears to play no part.
As most of the important sensitisers are briefly described under their respective names, refer ence should be made to these. The principal azo sensitisers are glycine red, benzonitrol brown, Pluto black, dianil black R, and wool black 4 B. To the rosaniline family belong ethyl violet and formyl violet. The phthaleine group is one of the most important, as it comprises the eosines and erythrosine. The acridine dyes are chrys aniline, acridine yellow, and acridine orange. The best sensitisers, particularly for red, belong to the chinoline or quinoline group, and are isochinoline red, dicyanine, and the isocyanines orthochrom T, pinaverdol, pinachrome, homocol, isocol, etc.