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DRY PLATES, FILMS AND PAPERS 165
the cyanin bands are shifted some distance towards the red and are at the same time drawn somewhat apart.
For certain unknown reasons the a band sometimes fails, and then, of course, the dye is of no use as a red sensitizer. Von Hubl suggests that the unsatisfactory action of such dyes in plates prepared with dyed emulsions as compared with that of bathed plates may perhaps be due to this.
Dicyanin 1 is a remarkable sensitizer in the extreme red when used with collodion emulsion. The sensitiveness is low and does not extend into the green, so that dark green light may be used with plates bathed in this dye.
Pinaverdol is especially adapted for sensitizing dry plates or films to orange, yellow and green.
In the case of orthochrome T the makers claim that the sensitizing action reaches to the C line of the spectrum, while pinachrome renders films sensitive to line B, and faster than those dyed with orthochrome.
Pinacyanol forms a blue solution when dissolved in alcohol, but the addition of water when the bath is prepared, changes the colour of this solution to purple. Like dicyanin, this dye does not sensitize for green rays, yet films dyed by pinacyanol are very fast and their sensitiveness extends far into the red.
Cyanin is a blue dye, and films dyed with it are sensitive far into the red. They are, however, very slow and are very liable to fog, hence great care must be taken to use suitable developers with plates sensitized by means of this dye. Many other dyes are used — e.g. Erythrosine, Rose Bengal, Eosin, Alizarin Blue S., etc., all of which have the power to increase the sensitiveness of the plates to the rays from the red end of the spectrum.
The theory of the action of dyes in this connection has been the subject of some amount of controversy. Vogel considered that coloured light is absorbed by the dye and that this light is transferred to the silver salt in the
1 W. A. Scoble, Photographic Journal, May 1906.