'It was often noted that with all four lantern images of equal luminosity, using intermission-frequencies too rapid to allow flicker, the brightness of the binocular combination of any two did not distinctly exceed that of the uniocular. In certain instances the binocular combination did appear just distinctly the brighter. This was for instance the case when of the four lantern images the two on the same horizontal level were combined by simple convergence. This excess of brightness is the well-known phenomenon examined by Jurin [note: 'Smith-Kästner, Lehrbegriff der Optik, 1755.'], Harris [note: 'Optiks, 1775.'] Fechner [note: 'Abhandlung d. Akad. Wiss. Leipzig, VII. 423, 1860.'], Aubert [note: 'Physiologie d. Netzhaut, S. 287, Breslau, 1865.'], Valerius [note: Poggendorff's Annal. Bd. 150, S. 17, 1873.'] and others.' (47)

'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.' (49)