It is proposed that the positive afterimage is generated by the energy given off by a substance which accumulates during exposure of the retina to light and dwindles away when the eye is kept in darkness. An attempt is made to show how this proposal is related to such phenomena as the latent period, the rate of rise and fall, the negative phase, the flight of colors, neutralization and the effect of the positive afterimage on the absolute threshold and binocular brightness matches.