20 PSYCHOLOGY. 



independent object on the surface, and are no longer de- 

 ceived in judging it to be something over which the color 

 of the ground is drawn. The physiological theory, on the 

 other hand, maintains that the contrast-effect is still pro- 

 duced, but that the conditions are such that the slight 

 changes in color and brightness which it occasions become 

 imperceT3tible. 



The two theories, stated thus broadly, may seem equally 

 plausible. Hering, however, has conclusively jDroved, by 

 experiments with after-images, that the process on one part 

 of the retina does modify that on neighboring portions, 

 under conditions where deception of judgment is impossi- 

 ble.* A careful examination of the facts of contrast will 

 show that its phenomena must be due to this cause. In all 

 the cases ichich one may investigate it ivill be seen that the up- 

 holders of the psychological theory have failed to conduct their 

 experimeiits ivith sufficient care. They have not excluded 

 successive contrast, have overlooked the changes due to 



* Hering : 'Zur Lehre vom Licbtsiune.'— Of these experiments the fol- 

 lowing (found on p. 34 ff.) may be cited as a topical one : "From dark 

 gray paper cut two strips 3-4 cm. long and | cm. wide, and lay them on a 

 backgioundof 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 i 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 gen- 

 eral 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 back- 

 ground, which is equally bright on both halves. Here can no longer be 

 any question of contrast-effect, because the conditio sine qua non of con- 

 trast, namely, the differing brightness of the ground, is no longer pres- 

 ent. 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 corre- 

 sponding portions of the retina, and from this follows further that bota 

 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." 



