650 



PHYSIOLOGY 



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. 295) is divided into 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 b and d for the observer's eyes. At + and + two 

 small grey crosses are fixed about the middle of the compartment on sheets 



Red 



a 



o 



c, 



Blue 

 glass 



Purple Yello 



Purple 



iW 



Green Purple 



Purple 

 FIG. 295. 



of transparent glass. On looking through the openings b 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 pre- 

 dominating. To the judgment, however, there is one background and not two, 

 and therefore, according to the theory of Helmholtz, the grey crosses should 

 by contrast both acquire the same induced colour, which would be comple- 

 mentary 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 simul- 

 taneous contrast are peripheral and not central in their causation. The same 

 fact is very definitdty established by the following experiment devised by 

 Sherrington. The disc (Fig. 296) presents two rings, each half-blue and half- 

 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 evidtnt 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 flickers long after complete fusion has taken place in the inner ring. 



