CH. LVIII.] SIMULTANEOUS AND SUCCESSIVE CONTRAST 845 



the Young-Helmholtz theory, this is explained on the hypothesis 

 that the excitation does not decline with equal rapidity in the three 

 colour terminals. A positive after-image is readily obtained by 

 momentarily looking at a bright object, e.g. a window, after waking 

 from sleep. Negative after-images may be seen either by closing 

 the eyes or by turning them to a uniform grey surface after viewing 

 an object steadily. If the object looked at is coloured, the negative 

 after-image seen upon such a background is in its complementary 

 colour; this is explained by the Young-Helmholtz theory, on the 

 supposition that the colour-perceiving element for the colour looked 

 at is the most fatigued, and the terminals for its complementary 

 colour least fatigued. On the Hering theory, one colour produces 

 anabolic or katabolic effects as the case may be ; on withdrawing the 

 eye from stimulation by that particular colour, the opposite phase of 

 metabolism takes place and produces the complementary colour. 



Simultaneous and Successive Contrast. Negative after-images are 

 frequently spoken of as phenomena of successive contrast. The 

 phenomena of simultaneous contrast are well illustrated by the 

 four figures of the accompanying Plate. In all these figures the 

 oblong grey strip is actually of the same brightness. This can easily 

 be proved by screening from view the surrounding parts of the 

 figures, which cause the greys to appear different. The grey in I. 

 appears darker than that in II., while the grey in III. appears 

 yellowish and in IV. reddish. If these effects are not sufficiently 

 obvious, they immediately become so when the entire surface is 

 covered over with a sheet of thin tissue paper. 



Figs. I. and II. are examples of brightness contrast ; Figs. III. and 

 IV. of -colour contrast. The effects of these two varieties of 

 simultaneous contrast may be stated thus : a given grey object looks 

 darker when viewed against a bright background than when viewed 

 against a dark background ; when the background is coloured, it is 

 tinged with the complementary colour of the former. 



Helmholtz attributed the effects of simultaneous contrast to 

 errors of judgment, and not to altered conditions of the retinal 

 apparatus.* But there can be no doubt that simultaneous contrast 

 has as simple a sensory origin as successive contrast (negative after- 

 images). For if either of the two lower figures of the plate is care- 

 fully fixated for about a minute (fixation of the central dot will 

 help to prevent involuntary movements of the eyes), and if the 

 gaze be then transferred to a spot on a sheet of white or grey paper, 

 not only will the outer squares appear in their complementary 

 colour, but also the grey strips will appear tinged, now likewise in a 



* By "retina" here and elsewhere we mean " cerebro-retinal apparatus." We 

 have no knowledge of the precise share of retina and brain in the development of 

 visual sensations and after-sensations. 



