A 4° square, two-part photometric field, symmetrical about a vertical division and viewed through a pupil 3 mm in diameter, is illuminated in both parts by artificial sunlight at a constant brightness of about 3 or 4 millilamberts (retinal illumination, 70 to 90 photons) with a surrounding field of about 0.5 millilambert. Homogeneous light is added to one-half, and sunlight simultaneously subtracted so that the field remains matched in brightness. Two adjustments of the mixture are made: (1) the least purity perceptible with certainty (pmax), and (2) the greatest imperceptible purity (pmin). The purity of these mixtures is then measured, increased accuracy being obtained by measuring a known large multiple of the homogeneous brightness. Values of pmax and pmin have been obtained as a function of the wave-length of the homogeneous component; these values are reported in detail, and some discussion of their interpretation is given.
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