CHAP, n.] SIGHT. 537 



may, in this case, be partly explained by the diffusion circles which, 

 in each case, encroach from the white upon the dark. But over 

 and beyond this, any sensation, coming from a given retinal area, 

 occupies a larger share of the field of vision, when the rest of the 

 retina and central visual apparatus are at rest, than when they are 

 simultaneously excited. It is as if the neighbouring, either retinal 

 or cerebral, structures were sympathetically thrown into action at 

 the same time. 



Contrast. If a white strip be placed between two black strips, 

 the edges of the white strip, near to the black, will appear whiter 

 than its median portion ; and if a white cross be placed on a black 

 background, the centre of the cross will appear sometimes so dim, 

 compared with the parts close to the black, as to seem shaded. 

 This occurs even when the object is well in focus ; the increased 

 sensation of light which causes the apparent greater whiteness of 

 the borders of the cross is the result of the 'contrast' with the 

 black placed immediately close to it. Still more curious results 

 are seen with coloured objects. If a small piece of grey paper be 

 placed on a sheet of 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. If a book, or pencil, be 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 complementary 

 to that o'f the candle-light which surrounds it. If the candle be re- 

 moved, 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, a 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. 



Filling up the Blind Spot. Though, as we have seen, that 

 part of the retina which corresponds to the entrance of the optic 



