io66 VISION. 



the explanation of Helmholtz. One eye looks through red and the other 

 through blue glass at a white surface, which appears violet, owing to binocular 

 colour mixture (p. 1124); a black patch on the white surface is doubled by 

 converging the eyes for a nearer point, and the double images seen against the 

 violet ground are coloured differently by contrast, — that seen by one eye being 

 blue-green, and that seen by the other, yellow. This experiment would show 

 that two separate illusions of judgment may occur simultaneously, one for each 

 eye. The occurrence of mixture of contrast colours is also in favour of their 

 physiological origin. 



A simple but striking experiment, illustrating both temporal and spatial 

 induction, is given by Hering. 1 Place two broad white strips of paper on 

 black velvet, so that a strip of the black, about 5 mm. wide, is left between 

 them. If a point is fixed on the black strip, this will be seen to become 

 lighter, and, on removing the white strips to one side, after fixation for thirty 

 seconds, the. black strip will become much brighter, and, with suitable adjust- 

 ment of the general illumination, may appear brighter than the objective white 

 strips. The surfaces previously covered by the white strip are black by suc- 

 cessive contrast, but the blackness does not seem great enough to account for 

 the brightness of the middle strip. The same appearance may be observed 

 with closed eyes. Hering regards the experiment as showing that the visual 

 organ, by its own inherent capacity, can produce a greater brightness in one 

 part of the field than is produced in another part by white light. 



This experiment is still more striking if coloured (red) strips on a black 

 ground are used. On removing the red strips, a red after-image is seen corre- 

 sponding to the black strip between them, and this occurs when the green colour 

 of the surrounding surface produced by successive contrast is hardly noticeable. 

 This appearance is obviously closely related to the corona of the after-image. 



According to Hering, the reciprocal relation of retinal areas is a 

 fact of the most fundamental importance in vision. Its function is to 

 counteract the effect of their regularly diffused light which surrounds 

 every retinal image, and the reciprocity is therefore most marked at 

 the junction of the reacting surfaces. That objects are seen with sharp 

 outlines is due to this reciprocal action, and to the production of a 

 retinal condition opposed in nature to the original excitation. Hering 

 regards it as incredible that a phenomenon of such fundamental 

 importance as contrast should not depend on some physiological mode 

 of relation between retinal (cerebro-retinal) areas. The nature of this 

 physiological relation will be discussed later. 



Plateau, 2 who advocated a physiological explanation of both after- 

 image and contrast, referred them to oscillations in time and space 

 respectively. The periodical variations of an after-image were examples 

 of the former kind of oscillation, while he regarded irradiation and 

 contrast as due to a similar oscillatory condition, passing outwards in 

 the retina from the place of stimulation. 



The phenomenon described on p. 1056, in which subliminal excitation in 

 a small retinal area is reinforced by stimulation of other areas of the retina 

 by the same kind of light, seems to be an instance of induction opposed 

 in nature to the ordinary form. It ceases, however, to be contradictory, 

 if one assumes that the opposed state induced in the retina by stimulation 

 of one part, reacts on the inducing surface. When a number of small 

 retinal areas are stimulated, the induced change will be more intense than in 

 the case of only one area, and the effect of the reaction may be sufficient 

 to raise the colour of the small areas above the threshold. 



1 Arch./, d. ges. Physiol., Bonn, 1888, Bd. xliii. S. 266. 



2 Bull. Acad. roxj. de mid. de Belg., Bruxelles, 1876, tome xlii. p. 684. 



