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Proceedings of the Royal Society of Edinburgh. [Sess. 
VI. — On Fechner’s Law and the Self-Luminosity of the Eye. 
By Professor William Peddie, D.Sc. 
(MS. received November 22, 1920. Read November 22, 1920.) 
(Abstract.) 
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. 
