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.