THE DURATION OF VISUAL SENSATIONS. The visual sensation upon the retina persists after the luminous stimulus has ceased; its duration is never less than one-tenth of a second, m) matter how short the luminous stimulns. The spokes( of H rapidly revolving wheel appear as a solid mass, because the images of the spokes upon the retina follow one another so rapidly that one impression cannot fade away before another has replaced it. For the same reason. a rapidly re volving disk upon which all the colors of the spectrum are represented gives the effect of a white surface, a second color sensation being thrown upon the retina before the impression of the first has had time to disappear.
This persistence of the retinal impression is responsible for posit ive after-images. if a brightly illuminated object be looked at. steadily, the impression upon the retina may be so per sistent that the form and color of the object are distinctly visible for a considerable time after the luminous stimulus has ceased to act. This positive after-image may be experienced by look ing at a bright object, such as a gas jet, for a short period and then closing the eyes, when a brilliant image will be perceived for some time.
A luminous impulse of very short duration is sufficient to produce a complete retinal impres sion; thus it has been calculated that an electric spark 20 000,000 of a second is distinctly visible. This short duration refers only to the time necessary to excite the sensibility of the retina; the time required for perceiving the sen sation is much longer, since the latter process in cludes passage of the visual sensatiou through the optic nerve (lasting at least of a sec ond), transmission through the intracerebral visual paths, and perception by the visual cortical area of the brain. Consequently by the time the image of an electric spark is perceived, it has already come to an end. This explains the fact that bodies in rapid motion appear as if at rest, when seen by a very brief illumination, such as an electric spark. For the same reason, the brief closure of the lids in winking causes no interference with sight, since the visual impres sion of external objects persists beyond the time required for rapidly closing and opening the lids. See DURATION.