Abstract
The basis of the widely held opinion that the spectral sensitivities of the mechanisms determining trichromacy must be linearly related to the spectral mixture functions is examined, and a proof from simple assumptions is stated. It is proved also that, on the van der Velden hypothesis of independent sensitive units, a spatial or temporal summation law of the form AIn = c implies a frequency-of-seeing curve of the form Q = I— e−KIn ( Q = probability of detecting a stimulus of size A and intensity I; n, c and K are constants).

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