Having traced the colourless after-image to 'higher sources' - that is, to the action of the blood - let us now examine coloured afterimages. We need first to become conscious of the colour-creating light-activity which resides in the blood. For this purpose we expose the eyes for a moment to an intense light, and then darken them for a sufficient time. Nothing in external nature resembles in beauty and radiance the play of colour which then arises, unless it be the colour phenomenon of the rainbow under exceptionally favourable circumstances.