[21] Loc. cit. p. 406.
[22] E. Hering, in Hermann's Handbuch d. Physiologie, iii. 1, p. 565.
[23] Hering: 'Zur Lehre vom Lichtsinne.'—Of these experiments the following (found on p. 24 ff.) may be cited as a typical one: "From dark gray paper cut two strips 3-4 cm. long and 1/2 cm. wide, and lay them on a background of which one half is white and the other half deep black, in such a way that one strip lies on each side of the border-line and parallel to it, and at least 1 cm. distant from it. Fixate 1/2 to 1 minute a point on the border-line between the strips. One strip appears much brighter than the other. Close and cover the eyes, and the negative after-image appears.... The difference in brightness of the strips in the after-image is in general much greater than it appeared in direct vision.... This difference in brightness of the strips by no means always increases and decreases with the difference in brightness of the two halves of the background.... A phase occurs in which the difference in brightness of the two halves of the background entirely disappears, and yet both after-images of the strips are still very clear, one of them brighter and one darker than the background, which is equally bright on both halves. Here can no longer be any question of contrast-effect, because the conditio sine quâ non of contrast, namely, the differing brightness of the ground, is no longer present. This proves that the different brightness of the after-images of the strips must have its ground in a different state of excitation of the corresponding portions of the retina, and from this follows further that both these portions of the retina were differently stimulated during the original observation; for the different after-effect demands here a different fore-effect.... In the original arrangement, the objectively similar strips appeared of different brightness, because both corresponding portions of the retina were truly differently excited."
[24] Helmholtz, Physiolog. Optik, p. 407.
[25] In Archiv f. d. ges. Physiol., Bd. xli. §. 1 ff.
[26] Helmholtz, loc. cit. p. 412.
[27] See Hering: Archiv f. d. ges. Physiol., Bd. xli. S. 358 ff.
[28] Hering: Archiv f. d. ges. Physiol., Bd. xl. S. 172 ff.; Delabarre: American Journal of Psychology, ii. 636.
[29] Hering: Archiv f. d. ges. Physiol., Bd. xli. S. 91 ff.
[30] Die Gesichtsempfindungen u. ihre Analyse, p. 128.