Abstract: Much work has already been done on this subject, though little of a quantitative character. Many observers have described the curious our sensations which rapid alternations of light and darkness can cite under certain conditions, admirably exemplified in the “spectrum tops.” Foremost amongst those whose experiments and writings have led to the present very general interest in the subjects of flicker, and of the sensation of light and colour, may be mentioned Helmholtz, vanus Thompson, Shelford Bidwell, Henry, Charpentier, and tod; whilst the first to try experiments on the relative sensitivess of the eye to flicker in light of different colours, seems to have been J. Plateau, who, however, employed pigments, and not the colours of the spectrum.