LXXIV.] 



CONTRAST. 



355 



than the rest of the same zone, and shades off gradually to the 

 outer part of the zone. 



(e.) Take two pieces of different coloured paper (say pale red and 

 pale green) and place them side by side. Fix two similar strips 

 apart from each other and 

 distant from the other two. 

 The two slips juxtaposed 

 differ in colour from the 

 isolated pieces. In the juxta- 

 posed slips the colour of 

 the one influences the colour 

 of the other, i.e., each one 

 looks as if it were mixed 

 with a certain amount of 

 the complementary colour 

 of the juxtaposed slip 

 (Clevreul). 



FIG. 276. Disc for Contrast. 



(/.) Place on a table a small 

 sheet (4" x 4") of red and one of 

 green paper. Cut out of a sheet 

 of red paper two pieces about 

 i inch square, and place them on the two large squares. Observe that the 

 small red square on the green ground appears far brighter and more saturated 

 than the red square on the red ground. 



(g.) Cut a small hole (5x5 mm.) in a piece of coloured paper, e.g., red, 

 and look through the hole at a sheet of white paper, the hole appears 

 greenish. 



(//.) On a mirror place a slip of transparent coloured glass, e.g., red or green. 

 Hold in front of the coloured glass a narrow strip of white paper ; by adjust- 

 ing the position of the glass in relation to the light, we see two images reflected 

 from the anterior and posterior surface of the mirror ; one has the same colour 

 as the coloured glass, while the other or posterior one has the complementary 

 colour ; if a red glass be used the latter is green, if a green glass it is red. 

 Hold in front of the red glass a piece of white paper with black printed matter 

 on it. The black print is seen green in the posterior image. Gum a few 

 narrow strips of white paper (i mm. in diameter) on black paper, and on hold- 

 ing it up in front of the red glass, as before, the anterior image appears in the 

 complementary colour of the glass, viz., green. 



(/.) Place four lighted candles in a dark room before a white surface, and 

 push between the candles and the screen towards the centre of the series an 

 opaque screen, e.g., cardboard, with a clean-cut vertical edge. A part of the 

 white surface is illuminated by all four candles, then a vertical area illumi- 

 nated by three, and so on, and finally a part not illuminated by any of the 

 candles. Each of these areas is throughout its entire extent equally illumi- 

 nated, yet on the side where each area abuts against a darker area it appears 

 lighter, on the other side darker, and gradually shaded between its outer and 

 inner limits. This is due to the fact that strong stimulation of one part of 

 the retina diminishes the excitability in the other parts, and the parts most 

 affected are those next the excited area. Thus a change in the excitability 

 of one part of the retina is brought about by stimulation of an adjacent 

 part. 



