AbstractThe additivity of flicker responses produced by two different wavelengths λ1and λ2was investigated for five adapting levels of 560 nm by the summation‐index method. A summation‐index value of σ of 0.30 was obtained for all conditions employed, confirming the linear property of the achromatic channel. The additivity of unique chromatic responses, red, yellow, green, and blue, was also investigated with the same method. In this case the wavelengths λ1and λ2were chosen in a spectral region to produce one of the four unique colors and the summation index was measured for the color with the cancellation method. A summation index of 0.30 was found for the green and blue sensations and also for the red sensation if both stimuli were from the same spectral region. A slight additivity failure of the enhancement type was found for pairs λ1and λ2λ1was in the short‐wavelength region while λ2was in the long‐wavelength region. An additivity failure of the same type was found for the yellow chromatic sensation when λ2was 607 nm and λ1was 570, 550, or 533 nm, the last combination giving a summation index of as much as 0.64. The results imply the existence of two different cones whose responses do not add linearly to yield th
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