§ 11. The Positive Afterimage, etc.—Light acting on the retina takes a certain time to produce its full effect, and the retinal excitement takes a certain time to disappear after the stimulus has been removed. If we take a black disk with a white sector, and set it in very rapid rotation, the whole disk appears to the eye as a uniform grey. As the white sector is whirled round, it affects successive portions of the retina, but by no means so intensely as if it continued to act on the same part. Owing to the rapidity of the rotation, it returns again to the same point before the effect of the previous stimulation has become appreciably diminished. The result is a uniform grey identical with, that which would be produced in the white light from the sector were equally distributed over the whole surface of the rotating disk at rest. The persistence of the visual sensation after the stimulus has ceased gives rise, under certain conditions, to what is known as the positive afterimage. To obtain this, the eye must briefly glance at an object, instead of steadfastly gazing at it. The conditions are most favourable when an eye which has for some time been withdrawn from the influence of light is momentarily exposed to a somewhat strong stimulus. ''Thus if immediately on waking from sleep in the morning the eye be directed to a window for an instant and then closed, an image of the window with its bright panes and darker sashes, the various parts being of the same colour as the object, will remain for an appreciable time."*