288 ELEMENTARY PHYSIOLOGY. [XXVII. 



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 cover- 

 ing 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 illuminated by reflection from a 

 white cloud, not with direct sunlight. On 

 the side of the paper opposite the window 

 place a lighted candle, and between it and 

 the paper place a book edge-ways, or any 

 object which will throw a shadow on the 

 paper. Between the paper and the window 

 place a similar object, throwing a like shadow. 

 The distance of the candle should be such 

 that the two shadows are of nearly equal 

 intensity. 



The shadow from the candle, though illumi- 

 nated by the white sunlight, will appear blue, 

 the complement of the reddish yellow colour 



