'JOH. MÜLLER described the elementary parts of the retina as being connected together by "reciprocal action [note: 'Muller. Handbuch der Physiologie des Menschen (1837-1840).']." The physiological result of application of a stimulus to any given point of a sensifacient surface is decided by not only the particular stimulus there and then incident but also by circumjacent and antecedent stimuli.' (33)

'At the basis of the phenomenon of simultaneous contrast lies, as Fechner [note: 'Poggendorff's Annal. XXXVII, XLIV. &c. 1837, &c.'], Mach [note: 'Sitzungsb. d. kais. Akad. d. Wiss. Wien, 1866 &c.'], and Hering [note: 'Sitzungsb. d. kais. Akad. d. Wiss. Wien, 1872 and later.'] may be said to have proved, a reciprocal physiological relation between units of the retino-cerebral apparatus such that activity of the apparatus connected with a point of retinal surface P affects the reactions of the apparatus belonging to the retinal area circumjacent about P. This reciprocity, subconscious in origin, affects consciousness; it is a factor in the production of sensations, and influences inferences dependent upon comparison of those sensations. So intrusive is it psychologically that Joh. Müller [note: Physiologie, 1841. Bk. V., Sect. 1, Cap. 3.'] and Helmholtz [note: Physiol. Optik. p. 417. Part II. 1860.'] treated it as wholly a product of processes of judgment, a doctrine ably controverted by Hering.' (38)