1921-22.] On Self Light, Fatigue, Inhibition, etc. 
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XX. — On Self Light, Fatigue, Inhibition, and Recurrent Visual 
Images. By Professor W. Peddie, D.Sc. 
(MS. received June 19, 1922. Read June 19, 1922.) 
(Abstract.) 
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. 
Helmholtz extended Fechner’s expression for the differential sensation 
in one colour to a combination of three so as to obtain the curve of 
differential sensitivity throughout the spectrum. He found remarkable 
agreement with experimental data, and used the results for the deter- 
mination of the absolute fundamentals. He also treated the problems of 
contrast colours and after images, the former being treated from the 
point of view of psychical considerations, thus avoiding more detailed 
formal discussion. This was essentially the stage to which the subject 
was developed at the time of his death, and at which, in the matter of 
formal development, it has largely remained since. But great experi- 
mental development has been made since that time, especially in connec- 
tion with the questions of fatigue, inhibition, contrast, after images, and 
