Fechner's law states that change of visual perceptivity is proportional to the fractional change in the intensity of the light. At weak intensities a term, regarded as constant, has to be added to the intensity of the external light on account of the self-luminosity of the eye. By integration over the whole stimulated part of the retina, Helmholtz obtained an expression for the perceptivity which agreed with observation in so far as the general nature of the relation between perceptivity and external stimulus is concerned. Close correspondence can be obtained by assuming that the self-luminosity term in Fechner's expression is itself a simple function of the external stimulus, rising rapidly to a maximum and thereafter slowly falling to a steady value.