320 
Proceedings of the Eoyal Society of Edinburgh. [Sess. 
recurrent vision. The evidence of the existence of cross influences 
amongst stimuli has become conclusive. 
The recognition of these makes possible the formal treatment of 
contrast and other effects. Fatigue is formally taken into account through 
the employment of the integrated form of Fechner’s law. The three 
integration constants (each of which may expressly contain as a term 
the corresponding self-light parameter) directly express the extent to 
which fatigue has developed. They are, of course, constant only under 
given conditions of illumination and its duration. They are the instan- 
taneous threshold values of the fundamental stimuli. Inhibition occurs 
whenever the external stimulus falls below the threshold value. In 
dependence upon the values of the various parameters, including those of 
the cross connections, a sensation may decay according to an exponential 
law, simple or complex, or it may have an exponentially decaying 
oscillator}^ value. In this way the oscillatory succession of .coloured 
after images receives formal explanation. The determination of the 
mechanism underlying that formulation is a matter for physiological 
investigation. 
The trichromatic theory, essentially complete to at least a first 
approximation in its formal development, is as firmly established as the 
electron theory, or the molecular theory, or the electromagnetic theory 
of light. 
{Issued separately August 10 , 1922 .) 
