VISUAL PERCEPTIONS. 



779 



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 " con- 

 trast " with the black placed adjoining it. Still more curious results are 

 seen with colored objects. If a small piece of gray paper be placed on a 

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

 gray paper will appear of a pink color, the complementary of the green. 



FIG. 177. 



This effect of contrast is far less striking, or even wholly absent, when the 

 small piece of paper is white instead of gray, 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 color. If a book or pencil be placed vertically on a sheet 

 of white paper, and illuminated on one side by the sun and the other by a 

 . candle, two shadows will be produced, one from the sun, which will be illu- 

 minated 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 color complementary 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, and 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. 



677. Filling up the blind spot. Though, as we have seen, that part of 

 the retina which corresponds to the entrance of the optic nerve is quite in- 

 sensible to light, we are conscious of no blank in the field of vision. When 

 in looking at a page of print we fix the visual axis so that some of the print 

 must fall on a blind spot, no gap is perceived. We could not expect to see 

 a black patch, because what we call black is the absence of the sensation of 

 light from structures which are sensitive to light ; we must have visual 

 organs to see black. But there are no visual organs in the blind spot, and 

 consequently we are in no way at all affected by the rays of light which fall 

 on it. There is in our subjective field of vision no gap corresponding to the 

 gap in the retinal image. We refer the sensations coming from two points 

 of the retina lying on opposite margins of the blind spot to two points lying 

 close together, since we have no indication of the space which separates 

 them. Concerning the effects which are produced when an object in the 

 field of view passes into the region of the blind spot there has been much 



