The <i>d</i>-log <i>t</i> curves of typical fast, medium, and slow emulsions when coated with oils fluorescing in the blue, near ultraviolet, and far ultraviolet respectively, have been studied in the region 900 to 5000 Angstroms by means of a vacuum spectrograph designed especially for photographic photometry, together with ordinary quartz spectrographs. The reciprocity and intermittency failures of the same emulsions, unoiled and when coated with the three oils, have been studied in the range 2300–5000A. The absorption and fluoresence spectra of the oils were also studied in the same range. It was found that the contrast obtained with oil coated plates is constant throughout that part of the spectrum in which the oil absorbs all of the incident light, and is equal to the contrast of the unoiled emulsion near the wave length of maximum intensity of fluorescence of the oil. As the oils studied absorb completely from their respective long wave limits down to at least 900A, the contrast obtained in the Schumann region was constant for a given oil-emulsion combination. This property greatly simplifies photographic photometry in the Schumann region, and enables one to assume the reciprocity law throughout that region when oil-emulsion combinations are used which have been found to obey the reciprocity law at the fluorescence maxima. Most of the secondary problems of photographic photometry are thus transferred from the difficult Schumann region to the near ultraviolet or visible where they can be more readily attacked. A device for varying intensities in the Schumann region is described, which is easily calibrated by means of the reciprocity law after the latter has been tested. A number of suggestions are made by which the use of fluorescent materials can be expected to simplify certain other problems of photometry.
GEORGE R. HARRISON and PHILIPA. LEIGHTON, "HOMOCHROMATIC SPECTROPHOTOMETRY IN THE EXTREME ULTRAVIOLET," J. Opt. Soc. Am. 20, 313-326 (1930)