The advance of photography : its history and modern applications (1911)

164 THE ADVANCE OF PHOTOGRAPHY properties of the more commonly used dyes will be of interest. > Some Dyes for Red Sensitizing. — The cyanin dyes are all useful as colouring agents for silver chloride and silver bromide, and they render these compounds sensitive to the rays of the red end of the spectrum.1 The absorption spectrum of all the cyanins is characterized by two well-marked bands, the one lying towards the red known as the a band, and that nearer the blue as the ft band. The relative distance between these two bands remains nearly constant under different conditions in solutions of the dyes and also in dyed dry films ; on the other hand, the form of the bands, their intensity, and their position in the spectrum, depends upon the nature of the medium in which the dyes are dissolved. Thus solutions in quinoline, alcohol, and benzene show an intense a band, but the ft band is only very faintly indicated. In dilute solutions in water, the ft band is broad, and the a band only appears as a light shadow. As the concentration is increased the ft band shows a tendency to spread out, either in both directions, or towards the blue end of the spectrum only. In the case of dyed dry films the bands appear of almost equal intensities, but dyed collodion films always show an approximation to the spectrum obtained with solutions in alcohol, while stained gelatine films are more like water solutions with respect to their action upon light. The bands seem to approach nearer the red end of the spectrum when a solvent of high dispersive power is used. It is of particular interest to note that all the C3Tanin dyes sensitize photographic plates in a similar manner to the effect shown by the absorption spectrum of the dyes in the dried films. On account of the high dispersive power of silver bromide, 1 Baron A. von Htibl, Journal of Photography, March 1906.