CHAPTER XXII. 



SIMULTANEOUS CONTRAST. VISUAL JUDGMENTS. 



In the foregoing discussion it has been tacitly assumed that the 

 only light which has anything to do with the visual sensations 

 which arise from stimulation of any particular part of the retina is 

 the light which actually falls on that part. As a matter of fact this 

 is not so. Strong stimulation of one area, as well as causing the 

 appropriate sensation there, also tends to produce sensations of the 

 opposite nature in the parts nearby. 

 Experiment 72. Take two pieces of the same paper, neutral grey 



in colour, and lay one on a white, and one on a black back- 



FlG. 49. To show simultaneous contrast. 



ground. The first appears darker and the second lighter than 

 before because of the contrast with the surrounding field. The 

 effect may be increased by veiling the whole with a white tissue 

 paper. 



Experiment 73. Arrange a white background with a horizontal 

 row of candles in front of it and thrust a screen in from one 

 side, part way between the two (Fig. 49). One large area 

 of the background (GH) receives none of the candlelight and one 

 (AB) receives light from all the candles. Between these there 

 are a number of vertical strips, on the first of "which (FG) the 

 light from only one candle falls, on the second the light from 

 two (EF), and so on. One would expect each of these strips to 



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