A sensation of light is recorded in the brain not only when the retina is flooded with incident light, but also when the latter is withdrawn. There appears a heterogeneous pattern of bright areas, known as the chaotic light or the self light of the retina. Although no stimulus is given to the eye, certain processes must continue in the retina which excite a sensation of light in the brain. This jumble of white or coloured impressions has no definite form in the field of vision, unless the eye has previously been directed on to a particular defined object. The brain concentration involved in this action causes the patch of self light to reproduce more or less in form and dimensions the object fixated. This representation of the object is known as an after image.