CHAP, in.] SIGHT. 127 



placed vertically on a sheet of white paper, and illuminated 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 illuminated 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 comple- 

 mentary to that of the candle-light which surrounds 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 complementary 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 covering 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 introduced between the features of 

 an object and our perception of them. But before we attempt to 

 point out the exact manner in which the effect is produced, it will 

 be convenient to turn to some other effects which are also some- 

 times spoken of as those of "contrast." 



782. After-images. Successive Contrast. As we have 

 already ( 748) 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 momen- 

 tarily exposed to a somewhat strong stimulus. Thus if imme- 

 diately 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 to 

 a stimulus, the sensation which follows the withdrawal of the 

 stimulus is of a different kind ; the result is what is called a 



