VISUAL JUDGMENTS. 179 



seem of even brightness throughout; actually however, as the 

 figure shows, each one appears lighter at the margin which is 

 next the darker area and darker at the other edge, which 

 borders on the more brightly lighted strip. 



Experiment 74. Lay the piece of grey paper on a background of 

 bright red. Cover them both with white tissue paper. The 

 grey area looks tinged with green-blue, the colour complemen- 

 tary to that of the background. Change the background 

 several times, using various brilliant colours, and note what 

 influence each has on the apparent colour of the neutral grey. 

 The phenomenon of simultaneous contrast is seen in countless 

 ways in ordinary life. Shadows on sand in the sunlight look bright 

 blue because of their yellow back-ground. So, too, places in a 

 brightly lighted room which are shaded from the artificial light 

 but faintly illumined by daylight appear to be blue, though as a 

 matter of fact the light falling on them is colourless, only because 

 of the yellow in the light on the surrounding areas of the field. 

 The principal, consciously or unconsciously, is used in art, in 

 posters and on the stage, to make the effect of vivid colour more 

 vivid or to modify the shade of fainter ones. It is not clear how 

 the phenomenon is brought about. It does not happen in the retina, 

 for simultaneous contrast has been shown for the blind spot, 

 where there is no retina at all. The modification must take place 

 somewhere on that part of the brain which is concerned with sight. 

 In this connection it is interesting to remember that there are 

 other instances of the activity of one sensory area influencing the 

 sensitivity of another. Sherrington showed the stimulation of 

 one part of a receptive field lowered the threshold of the response 

 to a succeeding stimulation of another part of the field, for the same 

 or for an allied reflex (Immediate Induction). 



Judgment of Space and Depth. External objects, no matter 

 what their shape, can only form flat images, images of two dimen- 

 sions, on the retina. In spite of this we see the objects in three 

 dimensions, as having depth as well as width and height, and it is 

 of interest to analyse some of the factors which contribute to this 

 judgment as well as those which have to do with another closely 

 allied to it, the judgment of distance. It is not practicable to do 

 in the class room many of the experiments on the factors concerned 



