One of the most curious affections of the eye is that, in virtue of
which it sees what are called _ocular spectra_, or accidental colours.
If we place a red wafer on a sheet of white paper, and, closing one
eye, keep the other directed for some time to the centre of the wafer,
then, if we turn the same eye to another part of the paper, we shall
see a green wafer, the colour of which will continue to grow fainter
and fainter, as we continue to look at it.
By using differently coloured wafers, we obtain the following results: