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 allow that there always exists after an impression of colour a temporary organic disposition to see the complementary hue, this does not suffice as an explanation of these cases; we have to conclude further that imagination, led by the usual run of our experience, is here a co-operant factor, and helps to determine whether the complementary tint shall be seen or not.

Misinterpretation of Form.

More complex and circumscribed associations take part in those errors which we occasionally commit respecting the particular form of objects.  This has already been touched on in dealing with artistic illusion.  The disposition of the eye to attribute solidity to a flat drawing is the more powerful in proportion to the familiarity of the form.  Thus, an outline drawing of a building is apt to stand out with special force.

Another curious illustration of this is the phenomenon known as the conversion of the concave mould or matrix of a medal into the corresponding convex relief.  If, says Helmholtz, the mould of a medal be illuminated by a light falling obliquely so as to produce strong shadows, and if we regard this with one eye, we easily fall into the illusion that it is the original raised design, illuminated from the opposite side.  As a matter of fact, the visual impression produced by a concave form with the light falling on one side, very closely resembles that produced by a corresponding convex form with the light falling on the other side.