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.

We must here briefly recall how this phenomenon is generally explained
on Newtonian lines. The starting-point is the assumption that the eye
becomes fatigued by gazing at the colour and gradually becomes
insensitive to it. According to Newton's theory, if an eye thus
affected looks at a white surface, the sum of all the colours comes
from there to meet it, while the eye has a reduced sensitivity to the
particular colour it has been gazing at. And so among the totality of
colours constituting the 'white' light, this one is more or less
non-existent for the eye. The remaining colours are then believed to
cause the contrasting colour-impression.

If we apply the common sense of the Hans Andersen child to this, we see
where it actually leads. For it says no less than this: as long as the
eye is in a normal condition, it tells us a lie about the world, for it