device of looking at a small bit of the image through a tube, but in
their ordinary colour. We may be said to fall into illusion here in so
far as we overlook the exact quality of the impression actually made on
the eye. This point will be touched on presently. Here I am concerned to
show that this habit of allowing for the coloured medium may, in its
turn, occasionally lead to plain and palpable illusion.

The most striking example of this error is to be met with among the
curious phenomena of colour-contrast already referred to. In many of
these cases the appearance of the contrasting colour is, as I have
observed, due to a temporary modification of the nervous substance. Yet
it is found that this organic factor does not wholly account for the
phenomena. For example, Meyer made the following experiment. He covered
a piece of green paper by a sheet of thin transparent white paper. The
colour of this double surface was, of course, a pale green. He then
introduced a scrap of grey paper between the two sheets, and found that,
instead of looking whitish as it really was, it looked rose-red.
Whatever the colour of the under sheet the grey scrap took the
complementary hue. If, however, the piece of grey paper is put outside
the thin sheet, it looks grey; and what is most remarkable is that when
a second piece is put outside, the scrap inside no longer wears the
complementary hue.

There is here evidently something more than a change of organic
conditions; there is an action of experience and suggestion. The reason
of our seeing the scrap rose-red in one case and neutral grey in
another, is that in the first instance we vividly represent to ourselves
that we are looking at it through a greenish veil (which is, of course,
a part of the illusion); for rose-red seen through a greenish medium
would, as a matter of fact, be light grey, as this scrap is. Even if we