OCULAR SPECTRA. 373 



of a red object appears green; because the sensibility of that 

 portion of the retina, on which the red image has been im- 

 pressed, 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 re- 

 tina, 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 in- 

 fluenced, in a considerable degree, by the vicinity of other 

 coloured objects, which modify the general sensibility of the 

 retina. When a white or gray object of small dimensions, 

 for instance, is viewed on a coloured ground, it generally 

 appears to assume a tint of the colour which is complemen- 

 tary 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 characters written on this kind of paper appeared 

 to him to be green; and that he was afterwards much sur- 

 prised at discovering that the ink employed was a pure 

 black, without any tinge of colour, and on closer examina- 

 tion he found that the marks were also black. The green 

 appearance of the letters, in this case, was an optical illu- 

 sion, arising from the tendency of the retina, which had 

 been strongly impressed with red light, to receive impres- 

 sions corresponding to the complementary colour, which is 

 green. 



A philosophical history of the illusions of the senses would 

 afford ample evidence that limits have been intentionally as- 

 signed to our powers of perception; but the subject is much 



* Any two colours which, when combined together, produce white light, 

 are said to be complementary to one another. 



