The early years of the nineteenth century saw the final establishment, with regard to the ordinary phenomena of light, of the wave theory. And, in those years, the man, Thomas Young (1807), whose work had been mainly effective towards the final acceptance of that theory, put forward a tentative hypothesis regarding the actions and phenomena of colour vision. It stated essentially that these phenomena were explainable most simply on the assumption that all colour sensations that we can experience are due to the existence of three fundamental colour sensations which can be called into play in various proportions by the action of external luminous stimuli falling upon the retina.