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. The physiological process which comes to consciousness in this way as an experience of vision is exactly the same as the process which gives us experiences of vision in dreams. There is indeed evidence that when one awakens in a brightly lit room out of vivid dreaming, one feels less dazzled than on waking from dreamless sleep. This indicates that in dream vision the blood in the eye is active, just as it is in waking vision. The only difference is that in waking consciousness the stimulus reaches the blood from outside, through the eye, whereas in dreams it comes from causes within the organism. The nature of these causes does not concern us here; it will be dealt with later. For the moment it suffices to establish the fact that our organism is supplied with a definite activity of forces which we experience as the appearance of certain images of vision, no matter from which side the stimulus comes. All vision, physiologically considered, is of the nature of dream vision; that is to say, we owe our day-waking sight to the fact that we are able to encounter the pictures of the outer world, brought to us by the light, with a dreaming of the corresponding after-images. Just as the simple light-dark after-image shows a reversal of light-values in relation to the external picture, so in the coloured afterimages there is a quite definite and opposite relationship of their colours to those of the original picture. Thus, if the eyes are exposed for some time to an impression of the colour red, and then directed to a neutral surface, not too brightly illuminated, one sees it covered with a glimmering green. In this way there is a reciprocal correspondence between the colour-pairs Red-Green, Yellow-Violet, Blue-Orange. To whichever of these six colours one exposes the eye, an after-image always appears of its contrast colour, forming with it a pair of opposites.