'It is doubtful... to me whether the slight excess in brightness of the binocular image over its two equal uniocular components is really explicable as summation of the intensities of the reactions at the corresponding spots of the two retinae. Valerius [note: 'Poggendorff's Annalen, Bd. CL. S. 117, 1873.'] measured the increase to be one-fifteenth of the brightness of the uniocular image. Aubert's [note: 'Physiologie d. Netzhaut, S. 286, Breslau, 1865.'] diagram gives it as less than one-thirtieth. Aubert says it is not perceptible with brightness greater than that of white paper in diffuse daylight indoors [note: 'Physiologische Optik S. 500, Leipzig, 1876']' (49)