The changes in sensitivity and contrast obtained when Cramer Contrast Process, Seed 23, and Wellington Super-Xtreme plates are coated with any one of five different fluorescent oils were studied in the region 2200-3800 A, for the purpose of improving the precision of photographic photometry in the region 2000-2400 A, and to investigate the feasibility of extending this method into the extreme ultraviolet. Characteristic curves are given for various plate-oil combinations, and some of the changes in sensitivity and contrast found are tabulated. The effect of thickness of coating was found to be great with most oils; certain oils gave the greatest improvement in sensitivity with thick coatings, and others with thin ones. The greatest sensitivity increase was found with a clear paraffin oil and contrast plates; the greatest contrast was found with a light lubricating oil and the same plates. A 400-fold gain in exposure time by oiling was found in certain cases. Some oils were found to increase the density produced by a certain intensity if the intensity exceeded a certain value, and to decrease it otherwise. This effect varies with wave length, so that it was found possible to decrease the sensitivity at longer wave lengths and increase it at shorter wave lengths, thus making the sensitivity curve of the plate more uniform throughout the spectrum. Although the chief effect with most oils is below 2400 A, certain varieties increase the contrast even up to 3800 A, although the sensitivity was generally decreased above 2500A. Oiled plates, when the oil coating is uniform, were found to be perfectly applicable to photographic photometry, accuracy to one per cent being readily attainable even at the shortest wave lengths studied. Sections of the “characteristic surface” in which the reciprocity law holds to within one per cent were found for several wave lengths, including 2200A.
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