'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.' (38)

Areal reciprocity, other things being equal, varies inversely with the distance of retinal separation of the reciprocal loci. This relation which in its broad features is obvious enough, has been established in detailed fashion by Mach's papers' [note: 'Sitz. d. kais. Akad. Wien, loc. cit.']. In the black tooth and in the white notch (Fig. 5) intensification by reciprocity between each and its surrounding field, although effective all over the tooth and all over the notch is maximal only at the free end of each. Followed from attached base to free end in each, the intensification gradually increases and culminates at the terminal border. The application of the stimulus, the blow, or if preferred as regards black, the decrement of stimulus, is more abrupt when the incidence of notch and tooth upon a retinal point begins with the free end of either. That its peculiarly high physiological efficiency is largely due to mere abruptness of impact is borne out by the fact, wbich I have found by actual experiment, that the mere angular length of the tooth or notch makes little difference, ceteris paribus, to the amount of flicker produced.' (51)