§ 5. The Retina's own Light.—In the total and continued absence of external light, there still exists a field of view which does not consist of mere darkness. Upon a background of medium grey, there are seen specks and clouds of colour. This is due to the fact that retinal elements are continually being stimulated by such internal processes as the circulation of the blood and the redistribution of heat. This internal stimulation is called the retina's own light (Eigenlicht der Retina). The sensation of black is not obtained in its purest form in the complete absence of external stimulation. It arises when the eye passes from objects which stimulate it, to some object which fails to stimulate it except in a slight degree.