A large number of experiments, from those of Hess and Pretori (1894) to the more recent studies of Heinemann (1955) and of Hurvich and Jameson (1966), have given support to the fact that simultaneous brightness contrast in a particular ("test") region depends in a systematic way on the luminance and the size of the contiguous "induction" region. As regards the corresponding physiological mechanisms, modern electrophysiological research (Ratliff, class="EmphasisTypeItalic ">Hart-line and Miller, 1963) seems to support Hering's theory which attributes the phenomenon of contrast to lateral inhibition processes taking place at a rather peripheral level of the nervous system.