582 



PHYSIOLOGY 



immediately adjoining that point. To this process the name of ' retinal induc- 

 tion ' has been given. In this way the phenomena of simultaneous contrast 

 may be explained. Thus if a ray of red light falls on any spot, it may be 

 supposed to excite dissimilation of the red-green substance at this spot. This 

 sets up assimilation of the same substance in the adjoining parts of the 

 retina, and the red object is therefore surrounded with a green halo, which 

 at once becomes evident if we increase our appreciation for slight colour- 

 tones by diminishing the total amount of light by means of tissue-paper. 



It has been much debated whether the contrast phenomena depend upon psychical 

 or retinal events. There is no doubt that the question must be answered in the latter 

 sense, and that these phenomena are quite independent of the judgment of the 

 individual. This is shown clearly by two experiments. A box (Fig. 297) is divided into 



Red 



Blue 

 glass 



Purple 



Purple 



+ 

 Yellow 



Purple 



FIG. 297. 



Green Purple 



two long compartments, a b and c d. At a the compartment is closed by a red glass - 

 plate and at c by a blue glass-plate. Apertures are provided at & and d for the observer's 

 eyes. At + and + two small grey crosses are fixed about the middle of the com- 

 partment on sheets of transparent glass. On looking through the openings 6 and d 

 and converging the eyeballs, so as to fix the line o, we get a fusion more or less complete 

 of the two colours red and blue, so that the background appears purple ; or there 

 may be a struggle between the colours, at one time blue, at another red predominating. 

 To the judgment, however, there is one background and not two, and therefore, accord- 

 ing to the theory of Helmholtz, the grey crosses should by contrast both acquire the 

 same induced colour, which would be complementary for purple. But it is found that 

 the two crosses are perfectly distinct in colour, that which is seen by the eye against 

 the blue ground being yellow while that on the red ground is green, showing that the 

 phenomena of simultaneous contrast are peripheral and not central in their causation. 

 The same fact is very definitely established by the following experiment devised by 

 Sherrington. The disc (Fig. 298) presents two rings, each half -blue and halt-black. 

 The outer ring is intensified when at rest by simultaneous contrast, the black half being 

 seen against the surrounding yellow, while the luminosity of the blue half is increased 

 by the effect of the surrounding black. In the inner ring the blue half is darkened 

 by contrast with the surrounding yellow, while the black half is not evident at all. 

 If the disc be rotated, we get two concentric rings on an apparently homogeneous 

 field. It is found, however, that the outer ring nickers long after complete fusion 

 has taken place in the inner ring, showing that the stimulation of the retina by the 

 outer ring is increased under the influence of contrast. 



