coloured after-image.

Only by representing the process in this way do we do justice to a fact
which completely eludes the onlooker-consciousness - namely, that the
eye produces the contrasting colour even while it is still exposed to
the influence of the outer colour. Since this is so, all colours
appearing to us in ordinary vision are already tinged by the subdued
light of the opposite colour, produced by the eye itself. One can
easily convince oneself of this through the following experiment.
Instead of directing the eye, after it has been exposed to a certain
colour, to a neutral surface, as previously, gaze at the appropriate
contrasting colour. (The first and second coloured surfaces should be
so arranged that the former is considerably smaller than the latter.)
Then, in the middle of the second surface (and in a field about the
size of the first), its own colour appears, with a strikingly
heightened intensity.

Here we find the eye producing, as usual, a contrast-colour from out of
itself, as an after-image, even while its gaze is fixed on the same
colour in the outer world. The heightened brilliance within the given
field is due to the addition of the after-image colour to the external
colour.

The reader may wonder why this phenomenon is not immediately adduced as
a decisive proof of the fallacy of the whole Newtonian theory of the
relation of 'white' light to the various colours. Although it does in
fact offer such a proof, we have good reason for not making this use of
it here. Throughout this book it is never our intention to enter into a
contest of explanations, or to defeat one explanation by another. How
little this would help will be obvious if we realize that research was