Cited by C.S. Sherrington, 'On Reciprocal Action in the Retina as studied by means of some Rotating Discs', Journal of Physiology 21 (1) (1897), pp. 33-54.
Description:

'The experiments described in this paper attempt to examine retinal "induction" by means of rotating discs. I am aware that in studying the subject thus by aid of moving chequered surfaces - that is, under intermittent excitation - time relations enter introducing complexity and preparing pitfalls for the observer. In the two well-known simultaneous contrast discs given by Helmholtz [note: Physiol. Optik. p. 413. Part II. 1860.'] the translation of surface is intended merely as a means towards the attainment of those smooth sensations - e.g. homogeneous grevs - which offer such favourable fields for contrast. The rate of translation used is one ensuring perfect fusion of the sectors.' (33-34)

 

'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)

 

Judgment based on comparison of the two blues Bl' and Bl and of the two blacks Bk and Bk' when the disc is stationary declares without hesitation that the difference between the blacks is much less than between the blues. Some observers fail to see any difference at all between the blacks, although they find a marked difference between the blues. Error of judgment thus, far from as in Helmholtz's doctrine [note: Physiol. Optik. 1860.'] producing the contrast, actually minimises it.

A similar instance of "Urtheilstäuschung" acting in exactly the converse manner to that suggested by Helmholtz - and, in regard to the important degree to which "Urtheilstäuschung" may affect observation, thorotughly bearing him out - is the following. Some persons unaccustomed to examine visual contrast have at first difficulty in admitting that there is a difference between the strips Bl and Bl' (Fig. 1). They have been told at outset that the disc is prepared from a piece of evenly-tinted blue card and the unpainted even-blue back of the card has been shown to them, and they then on looking at the face of the disc frequently have told me that the two strips appear of quite equal tint in saturation and in every other respect. On rotating the disc before them they however like others point out without hesitation that one ring-band is "flickering" while the other is "steady." This shows how in them the rôle of judgment is exactly the converse to that allotted to it by Helmholtz in his theory of "simultaneous contrast." They are really subjected like otlher observers to very different excitations from Bl and Bl'; yet, in all good faith, after hearing that the two bands are made of the same piece of evenly-tinted card they assert that their sensations derived from the two are not unequal. Their judgment suppresses the actual inequality of their sensations, and does so without their desiring that it should. Their judgment warped by their previous knowledge leads them to ignore the simultaneous contrast which is really taking effect in them - that it is really taking effect in them is proved by their acknowledging the actuality of areal reciprocity in themselves by acknowledging the "flicker" difference which is as patent to them as to others. The knowledge and idea of the physical equality of the bands acts in them much as does suggestion on a hypnotic subject. Because they know two things are the same they cannot see a difference between them.' (41-42)

Cited by D. Cahan, 'Helmholtz in Gilded-Age America: The International Electrical Congress of 1893 and the Relations of Science and Technology', Annals of Science 67 (1) (2010), pp. 1-38.