For colour vision, the task of the eye is to discriminate different distributions of energy over the spectrum. This is usually treated as a problem in the wavelength domain, analogous to treating spatial resolution in terms of spatial positions in the image. What is attempted here is a treatment of colour vision in terms of the system's responses to spectral energy distributions that are sinusoidal functions of wavelength. These are called comb-filtered spectra, and the treatment is analogous to that of spatial vision in terms of spatial sinusoids. This gives some insight into the reasons for trichromacy, the advantages of oil droplets, and the narrow separation of the red and green mechanisms. It is also shown that the absorption spectra of photosensitive pigments are superimposable if plotted as a function of the fourth root of wavelength.