VISION 125 



surface will appear much darker than that of the other 

 strip. (The difference of brightness between the two is 

 often more apparent in the after-image than in the 

 original strips.) 



b. Place three candles in front of a white, otherwise un-illumi- 

 nated surface; pass between them and the surface an opaque body 

 with a sharp clean-cut edge, so that part of the surface is illuminated 

 by the three candles, part by two, part by one, and part un-illumi- 

 nated; stand two or three yards back and look fixedly at the junc- 

 tion lines of the variously illuminated surfaces; it will be seen that 

 each -area is lighter close to a darker surface and darker close to a 

 lighter one than it is elsewhere. 



19. Simultaneous contrast, a. Cut out a thin cross 

 of grey paper, and place it in the middle of a sheet of 

 bright green paper. Cover the whole with a sheet of 

 thin tissue-paper. The grey patch will appear pink. The 

 exact tint of the patch will depend on the tint of the 

 green, of which it will be the complementary colour. 



Surround the grey cross with a broad, dark black rim. 

 The effect of contrast will be lost; the grey patch will 

 appear grey. 



On a red ground the grey cross will appear green, and with the 

 other colours similar complementary effects will be produced; but 

 the results are most striking in the case of red and green. 



The effect is greatest when the patch is grey, not white, and is 

 always heightened by covering with tissue-paper. 



b. Cut a thin strip of grey paper and place it across the junction 

 of a red with a green paper, and cover with tissue-paper. 



The grey will appear green on the red side and pink on the green 

 side. 



c. Place a sheet of white paper on a table before a window illumi- 

 nated by reflection from a white cloud, not with direct sunlight. On 

 the side of the paper opposite the window place a lighted candle, 



