Chap, hi.] SIGHT. 933 



sensation of light which causes the apparent greater whiteness 

 of the borders of the cross is regarded as the result of the ' con- 

 trast ' with the black placed immediately close to it. Still more 

 striking results are seen with coloured objects. If a book, or 

 pencil, be placed vertically on a sheet of white paper, and illumi- 

 nated on one side by the sun, and on the other by a candle, two 

 shadows will be produced, one from the sun which will be illumi- 

 nated by the yellowish light of the candle, and the other from 

 the candle which will in turn be illuminated by the white light 

 of the sun. The former naturally appears yellow ; the latter, 

 however, appears not white but blue ; it assumes, by contrast, a 

 colour complementary to that of the candle-light which sur- 

 rounds it. If the candle be removed, or its light shut off by a 

 screen, the blue tint disappears, but returns when the candle is 

 again allowed to produce its shadow. If, before the candle is 

 brought back, vision be directed through a narrow blackened tube 

 at some part falling entirely within the area of what will be the 

 candle's shadow, the area, which in the absence of the candle 

 appears white, will continue to appear white when the candle is 

 made to cast its shadow, and it is not until the direction of the 

 tube is changed so as to cover part of the ground outside the 

 shadow, as well as part of the shadow, that the latter assumes its 

 blue tint. If a small piece of grey paper be placed on a sheet 

 of pale green paper, and both covered with a sheet of thin tissue 

 paper, the grey paper will appear of a pink colour, the comple- 

 mentary of the green. This effect of contrast is far less striking, 

 or even wholly absent, when the small piece of paper is white 

 instead of grey, and generally disappears when the thin cover- 

 ing of tissue paper is removed. It also vanishes if a bold broad 

 black line be drawn round the small piece of paper, so as to 

 isolate it from the ground colour. And many other instances 

 of this kind of contrast might be given. It is obvious that 

 whenever in vision this effect intervenes, a discrepancy is intro- 

 duced between the features of an object and our perception of 

 them. 



§ 584. After-images. Successive Contrast. As we have al- 

 ready (§ 551) seen the visual sensation lasts much longer than 

 the stimulus, and under certain circumstances the sensation 

 is so prolonged that it is spoken of as an after-image. Such 

 after-images are best developed when an eye, which has for some 

 time been removed from the influence of light, is momentarily 

 exposed to a somewhat strong stimulus. Thus if immediately 

 on waking from sleep in the morning the eye be directed to a 

 window for an instant and then closed, an image of the window 

 with its bright panes and darker sashes, the various parts being 

 of the same colour as the object, will remain for an appreciable 

 time. 



When, however, the eye has been for some time subjected 



