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visibility of those rays to which the area has not been fatigued. An 

 after-image seen in complete darkness is ascribed to visibility of part 

 of the intrinsic light of the retina. 



The other view, advocated by Plateau and Hering, ascribes a more 

 active role to the retina, and supposes that a condition is set up 

 antagonistic in nature to the original excitation. Plateau supposed that 

 the variations of an after-image were due to an oscillatory process in the 

 retina on its way to a state of rest. 



The great difficulty of the former view lies in explaining the bright- 

 ness of the after-images which may be seen in complete darkness. The 

 brilliancy of these, and especially of coloured images, may be very 

 considerable. On the fatigue hypothesis, the latter are explained as 

 due to part of the intrinsic light, that part corresponding to the exciting 

 rays being absent. The brightness of such an image is, however, much 

 greater than that of the whole intrinsic light, even after prolonged stay 

 in the dark. That the brightness of an image should be greater than 

 that light of which it is a part, would seem an insuperable difficulty to 

 the fatigue hypothesis, but Helmholtz brought in the influence of 

 contrast to explain it. We have nothing with which to compare the 

 intrinsic light of the retina, while the after-image is seen against the 

 darker background corresponding to the rest of the retina, and its 

 apparent brightness is heightened by contrast, i.e. the phenomenon is 

 explained on psychological grounds. 



Another objection to the fatigue hypothesis is, that after-images are 

 observed most readily in the early morning, when the liability to fatigue 

 should be slight. They also occur especially well in the young and 

 vigorous. The periodical variations of after-images cannot be explained 

 by fatigue, especially the appearance of a positive image between two 

 negative phases. 



Hering has devised a number of experiments directed against the fatigue 

 explanation. 1 If a small saturated red disc on a somewhat darker grey ground 

 is fixed, and after an interval both disc and ground are shaded, the disc will 

 appear bluish green and much brighter than the ground. 2 In this experiment 

 the retinal area in question is still being objectively stimulated by red light. 

 The blue-green after-image would on the fatigue hypothesis be due to the 

 visibility of part of the small amount of white light reflected by the red disc. 



Hering has compared an after-image with objective light of the same 

 colour. On the white half of a black and white surface he placed a small red 

 disc 5 mm. from the line of junction of the black and white halves ; on the 

 black ground, 10 cm. from this line, a blue-green disc. After fixation of the 

 line of junction close to the red disc for one minute, he fixed a point on the 

 black surface 5 mm. from the blue-green disc, so that the subjective blue-green 

 image could be compared with the objective blue-green disc. With suitable 

 general illumination the after-image appeared brighter and more saturated than 

 the disc. Here an area of the retina " fatigued " to red produces a brighter 

 and more saturated blue-green sensation than one previously allowed to rest by 

 exposure for one minute to a black surface, and then stimulated by blue-green 

 light. According to Hering, this can only be due to some inherent capacity of 

 the visual organ. The nature of this capacity must be left till his special 

 theory is considered. 



1 For these and numerous other after-image and contrast experiments, see Arch. /. d. 

 ges. Physiol., Bonn, Bd. xliii. S. 264 and 329. 



2 This experiment was also carried out by Plateau, to whom it was suggested by 

 Wheatstone (Bull. Acad. roy. de med. de JSelg., Bruxelles, 1875, tome xxxix. p. 100). 



