. Physiological optics : being an essay contributed to the American encyclopedia of ophthalmology . o us: for each intensity the unit chosenhas been the minimum perceptible brightness for each color after theeye has been obscured in darkness for twenty-five minutes. 81 Values of —I Luminous Intensity Red Yellow Green Blue units .... .... .... 25 Do •••• •*•• •••• 100 225 400 900 233 PHYSIOLOGICAL OPTICS 203. These results are plotted in Fig. 120; one sees, with the unit ofluminous intensit
. Physiological optics : being an essay contributed to the American encyclopedia of ophthalmology . o us: for each intensity the unit chosenhas been the minimum perceptible brightness for each color after theeye has been obscured in darkness for twenty-five minutes. 81 Values of —I Luminous Intensity Red Yellow Green Blue units .... .... .... 25 Do •••• •*•• •••• 100 225 400 900 233 PHYSIOLOGICAL OPTICS 203. These results are plotted in Fig. 120; one sees, with the unit ofluminous intensity chosen, that the curves are very different. But thisis no longer the case if one takes as the basis the threshold values ofthe color sensation, that is the chromatic minimum, rather than theminimum light-sense values. For the photochromatic interval increasesfrom the red toward the violet; the result will be that one will obtainthe same curve for all colors within the limits of light intensities com-monly employed. Hence the differential sensibility is the same for ISO. loo Too ^foo mw Qoo Fig. 120.—Curves Illustrating the Light Sense with Various Colors and Inten-sities. (After Charpentier.) all the colors when one compares them under equal intensities, theunit being defined for each of them by its chromatic minimum. Withdecreasing luminous intensities the sensibility diminishes more rapidlyfor waves of longer length than for those of shorter length. Koenigand Brodhun have carried out an elaborate series of experiments inthis field. They determined the least perceptible brightness incrementfor lights of various colors, including white, for brightness of aneutral tint (white) surface illuminated to intensities varying from1,000,000 meter-candles to nearly the threshold of vision using anartificial pupil of 1 square millimeter area. 234 PHYSIOLOGICAL OPTICS 204. The essential features of a spectrophotometer similar to thatemployed by Koenig and Brodhu
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