ance
which powerfully suggests a particular arrangement in space. One of the
most striking examples of these is the erroneous localization of a
quality in space, that is to say, the reference of it to an object
nearer, or further off than the right one. Thus, when we look through a
piece of yellow glass at a dull, wintry landscape, we are disposed to
imagine that we are looking at a sunny scene of preternatural warmth. A
moment's reflection would tell us that the yellow tint, with which the
objects appear to be suffused, comes from the presence of the glass;
yet, in spite of this, the illusion persists with a curious force. The
explanation is, of course, that the circumstances are exceptional, that
in a vast majority of cases the impression of colour belongs to the
object and not to an intervening medium,[42] and that consequently we
tend to ignore the glass, and to refer the colour to the objects
themselves.

When, however, the fact of the existence of a coloured medium is
distinctly present to the mind, we easily learn to allow for this, and
to recognize one coloured surface correctly through a recognized medium.
Thus, we appear to ourselves to see the reflected images of the wall,
etc., of a room, in a bright mahogany table, not suffused with a reddish
yellow tint, as they actually are--and may be seen to be by the simple
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