1. The following work is based directly on the mathematical treatment of the phenomena of after images given in Colour Vision, a recent book on the subject, by Professor Peddie.

The aim is to prove that this treatment agrees with the results obtained from experiments in which one eye was exposed to light for a given period. The oscillating decay of the resulting after images may be represented by the combined effect of three exponentially decaying periodic curves representing, respectively, the sensations of red, of blue, and of green.