Investigations and Determinations of Wavelengths in the Red and Infrared Regions of the Spectrum

(K. W. Meissner. Untersuchungen und Wellenlängenbestimmungen im roten u. infraroten Spektralbezirk. Ann. d. Physik. 50 p. 713, 1916.)

Through the work of Vogel, Abney, Eder, and others,¹ as early as the eighties a method had been developed for shifting the sensitivity of a photographic plate to any region of the visible spectrum. For this purpose the plates are treated with solutions of cyanine, eosin, and other dyes, depending on the desired zone of sensitivity. With the appearance on the market of the dyes Dicyanin and Dicyanin A (Höchst a. M.), it became possible to extend the limit of plate sensitivity comparatively far into the infrared region. K. W. Meissner developed a fairly simple method of sensitization, making it possible to photograph the entire red region, at least as far as \(930\ \mu\mu\). A solution of the dye in boiling absolute alcohol (0.1 gr. per 100 ccm.) is poured, in an amount of 3 ccm., into a mixture of 50 ccm. of ordinary alcohol, 50 ccm. of distilled water, and 6 ccm. of a strong ammonia solution. Into such a bath, maintained at a temperature of \(20—22^\circ\), the plate is immersed for 3 minutes. Rapidity of drying is a necessary condition for sensitivity; to accelerate the process, the glass side of the plate is wiped dry, the plate is placed on the lid of a tin box with water at a temperature of \(35^\circ\), and dry air (temperature \(22^\circ\)) is driven over it by a fan. A plate prepared in this way reaches its maximum sensitivity only approximately on the second day and can still be used for another 2–3 days, after which the sensitivity declines and fog appears. Development may be carried out with a Glycin solution (1:10) for 20–30 minutes, depending on the exposure time. The presence of the sensitizer has a rather harmful effect in that, during development, not only the illuminated grains are blackened, but also grains accidentally located nearby, as a result of which the photographed spectral lines become somewhat thickened. This defect can be eliminated by washing the plate after sensitization in an equal solution of alcohol and water; in this case the gelatin layer is cleansed of excess dye. The most suitable plates for sensitization proved to be Wratten & Wainraight (London) plates; Hauff plates gave poor results. For photographing fine spectral lines, a similar method, with certain modifications, may be used to treat lantern-slide plates, which have a finer grain.

S. Vavilov.

¹ Cf. Abney. A treatise on photography 1893; J. M. Eder. Ausführliches Handbuch der Photographie, etc.