Sixty-four colors well-distributed near the Munsell 6-plane were viewed as 2° squares surrounded by a 5° black square which was in turn surrounded by a large area of Munsell N6/. The colors were viewed in random order at luminances of −3.34, −2.86, −2.37, −1.88, −1.38, and −0.88 log candles/m2, the illumination being at a color-temperature of about 6500°K. The observer was required to signal by means of a switch whether he observed red, orange, yellow, green, blue, purple, or grey. No time limit was imposed. Sixteen sets of observations were made by each of two observers, eight sets by a third observer. All had normal color vision. At the lowest luminance only occasional observations of purple were reported, nearly all colors appearing grey. As the luminance was raised, purple appeared first, then blue. The colors along an axis from about 7.5GY to 2.5P required the greatest illumination to appear chromatic. These results show an interesting correspondence with those of Middleton and Holmes on the appearance of colors subtending small angles.
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