A rationale and methodology is developed which allows the systematic and definitive testing of a scalar luminance-additivity law (Abney’s law) within any photometric system which operationally defines luminance equality. For example, under the postulate that all stimuli have the same luminance at absolute threshold, the following inference is made: If a subthreshold field of λc is set so that it is some proportion p of the energy required to bring itself to a just-visible level, then the energy of any other wavelength λi which must be added to the same λc subthreshold field in order to bring the λc+λi mixture to threshold should be the proportion 1−p of the energy required to bring λi to threshold when no mixing is involved.
In an experiment concerned with such mixtures, it is found that a scalar additivity law does not even approximate the facts of luminance addition at threshold, with complete failure of additivity being apparent under some conditions. Moreover, the data suggest the presence of an inhibitory effect in which a light is less visible when combined with subthreshold light of a different wavelength than when presented alone.
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