After-images

Those which continue after one has ceased to look at a bright object. They were first described by Aristotle, and have been investigated by many eminent men, including Augustine, Newton, Buffon, Erasmus, Darwin, Goethe, Fechner and Helmholtz. The image which remains may, at first, be similar to the original object in brightne and color, in which case it is called positive; thus a rocket or a meteor leaves a trail of light, and the positions of a trotting-horse are fused into a continuous whole. But after the eye has been exposed to a bright light, the image is likely to be the opposite of the original; thus the image of the sun is seen as a dark circle, and this is called negative. When the object is colored the image is apt to be of a complementary color, the image of a red object, e.g., appearing green: this is called complementary.