In an earlier note communicated last session the action of self light and fatigue was partially considered. The question of the origin of these effects is one which can only be settled by observation or experiment. The question of the mathematical form of the laws regulating the effects also requires an observational or experimental basis. In the absence of self light, Fechner's well-known law that the change in sensation is proportional to the fractional change in the stimulus forms a close approximation to results of experience throughout a wide range of intensity. Fechner's modification of this law, by the addition of a constant term to the denominator of the fraction, was shown by Helmholtz to account broadly for the action of self light; and he showed also that the introduction of three such constants into three such independent fractions enabled a good description to be given of various phenomena of colour vision in light of different intensities. But, from the phenomena of vision, it is certain that the numerical values of the parameters are influenced by antecedent as well as present illumination. Thus the parameters are really functions which are variable in more than one way, and the form of these functions can only be found by observation. This is the subject of the note referred to above.