COLOURS PRODUCED BY THE UNEQUAL ACTION OF LIGHT UPON THE EYES.
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.