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Sensitivity to luminance differences was determined under various experimental conditions as far as diameter of the test-spot and fixation modalities are concerned. Binocular performance is found to be significantly greater than the expected performance, obtained by applying the law of independent probabilities to monocular chances of perception, right and left respectively. The amount of the above said 'gain' varies when passing from small to large test spots, in a manner which cannot be strictly related to the decrease in monocular sensitivity occurring when the area of the test spot is increased. In addition, if fixation is so rigorous and the gaze so steady that a periodical black-out of the adapting field occurs, the 'gain' varies, when the diameter of the test-spot is varied, as expected on the basis of a previous finding according to which perception of small spots is imparied by incoming fading, while perception of large spots is improved. (Author)