472 THE SENSORIAL FUNCTIONS. 



appears green ; because the sensibility of that por- 

 tion of the retina, on which the red image has been 

 impressed, is impaired with regard to the red rays, 

 while the yellow and the blue rays still continue to 

 produce their usual effect; and these, by combining 

 their influence, produce the impression of green. 

 For a similar reason, the spectrum of a green 

 object is red ; the rays of that colour being those 

 which alone retain their power of fully impressing 

 the retina, previously rendered less sensible to the 

 yellow and the blue rays composing the green light 

 it had received from the object viewed. 



The judgments we form of the colours of bodies 

 are influenced, in a considerable degree, by the 

 vicinity of other coloured objects, which modify the 

 general sensibility of the retina. When a white or 

 grey object of small dimensions, for instance, is 

 viewed on a coloured ground, it generally appears 

 to assume a tint of the colour which is comple- 

 mentary to that of the ground itself.* It is the 

 etiquette among the Chinese, in all their epistles of 

 ceremony, to employ paper of a bright scarlet hue; 

 and I am informed by Sir George Staunton, that 

 for a long time after his arrival in China, the cha- 

 racters written on this kind of paper appeared to 

 him to be green ; and that he was afterwards much 

 surprised at discovering that the ink employed was 

 a pure black, without any tinge of colour ; and on 

 closer examination he found that the marks were 

 also black. The green appearance of the letters, 

 in this case, was an optical illusion, arising from 



* Any two colours which, when combined together, produce 

 white hght, are said to be complementary to one another. 



