572 



PHYSIOLOGY 



surfaces, or the demarcation of the junction of the two surfaces by means 

 of a narrow black or white line, or even the existence of small marks or 

 creases, reduces the effects of contrast to a considerable extent. 



We can summarise the effects of simultaneous contrast in the following 

 way : if one part of the retina is being stimulated, the part surrounding it 

 not only tends to discourage a similar stimulus but also favours one of a 

 complementary nature (black in this connection being considered as being 

 complementary to white). But this statement is similar to that which we 

 have already made with regard to the effects of the after image on the same 

 part of the retina which has received stimulation. Simultaneous contrast 

 would therefore appear to be simply an extension of the after image phenome- 

 non into a region of the retina outside the confines of the original stimulus. 

 Experiment shows that this extension does not go far from the excited area, 

 for the contrast effects, which may be' considerable near the contour, 

 rapidly decrease as the distance from the contour increases. 



BIDWELL'S EXPERIMENT. If a short white light stimulus be caused 

 to fall during the after image period of a previous red stimulus, we 

 should expect the white light to be tinted blue-green, because red light 

 is suppressed and its complementary increased. If the intensity and tinu> 

 intervals are carefully chosen, the blue- green sensation may be made 

 stronger than the original red stimulus to which it owes its colour. The 

 persistence of vision causes this blue-green response to last an appreciable 

 time, and therefore if another red stimulus rapidly succeeds the white one 



WHITE 



BLUE 

 GREEN 



ROTATING OlSC 



OBJECT LOOKED AT 

 BLUE GRtLEH 



"RED 



WHEN 

 THROUGH ROTATING OlSC. 



Fia. 294. Bidwell's rotating disc; an object looked 

 through it appears in complementary colours. 



(the one which is coloured blue-green) it will tend to be'suppressed from con- 

 sciousness. But the white stimulus succeeding this second red one will he 



