Accepting the additivity of luminosities in spectral color-mixture throughout a wide range of luminosities as an experimentally established principle, certain definite conclusions can be drawn concerning the constitution of the visual sensory mechanism. It is shown that a trisensational sensory system could satisfy the additivity of luminosities exactly only in the event of a direct-proportionality relationship between visual sensation intensity and stimulus intensity, unless the luminosity coefficients of two of the three sensations were sensibly null. However, in a trisensational system in which sensation intensity obeys Fechner’s law and more than one of the sensation luminosity coefficients has an appreciable value, the luminosity additivity law might hold to within the limit of observational error, but only if the spectral excitation curves of the elementary color-sensations overlap much more extensively than has usually been assumed. The form of surfaces of constant luminosity in a color-space representing a sensory system of the latter type is investigated. A sensory system in which luminosity is associated exclusively with a single element of the system would satisfy the additivity of luminosities unconditionally.
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