If we hold a slip of white paper vertically, about a foot from the
eye, and direct both eyes to an object at some distance beyond it, so
as to see the slip of paper double, then, when a candle is brought
near the right eye, so as to act strongly upon it, while the left
eye is protected from its light, the left-hand slip of paper will be
of a tolerably bright _green_ colour, while the right-hand slip of
paper, seen by the left eye, will be of a red colour. If the one image
overlaps the other, the colour of the overlapping parts will be white,
arising from a mixture of the complementary red and green. When equal
candles are held equally near to each eye, each of the images of the
slip of paper is white. If, when the paper is seen red and green by
holding the candle to the right eye, we quickly take it to the left
eye, we shall find that the left image of the slip of paper gradually
changes from _green_ to _red_, and the right one from _red_ to _green_,
both of them having the same tint during the time that the change is
going on.