COMPLEMENTARY COLOURS I COLOURED SHADOWS. 439 



particularly noticed in regard to the complementary^ colours, 

 especially red and green ; so that such persons are not able to 

 distinguish ripe cherries amongst the leaves of the tree, except 

 by their form. 



569. When the retina has been exposed for some time to a 

 strong impression of some particular kind, it seems less sus- 

 ceptible of feebler impressions of the same kind ; just as the 

 ear, when it has been exposed to a series of very loud sounds 

 (as the discharge of cannon in a naval engagement), is for 

 some time deaf to fainter ones. Hence several curious visual 

 phenomena result. If we look at any brightly luminous 

 object, and then turn our eyes on a sheet of white paper, we 

 shall perceive a dark spot upon it ; the portion of the retina 

 which had been affected by the bright image, not being affected 

 by the fainter rays reflected by that part of the paper. If the 

 eye has received a strong impression from a coloured object, 

 the spot afterwards seen exhibits the complementary colour ; 

 thus, if the eye be fixed for any length of time upon a bright 

 red spot on a white ground, and then be suddenly turned so 

 as to rest upon the white surface, we see a green image of the 

 spot. The same explanation applies to the curious pheno- 

 menon of coloured shadows. It may be not unfrequently 

 observed at sunset, that, when the light of the sun acquires 

 a bright orange colour from the hue of the clouds through 

 which it passes, the shadows cast by it have a blue tint. 

 Again, in a room with red curtains, the light which passes 

 through these produces green shadows. In both instances, a 

 strong impression of one colour is made upon the general 

 surface of the retina ; and at any particular spots from which 

 the coloured light is excluded, that particular hue is not per- 

 ceived in the faint light that remains, and its complement 

 only is visible. The correctness of this explanation is proved 

 by the fact, that, if the shadow be viewed through a tube, in 

 such a manner that the coloured ground is excluded, it seems 

 like an ordinary shadow. 



1 White, or colourless light, is spoken of as composed of three primary 

 colours, red, blue, and yellow. By the complementary colour is meant 

 that which would be required to make white light, when mixed with 

 the original. Thus, red is the complement of green (which is composed 

 of yellow and blue) ; blue is the complement of orange (red and 

 yellow); yellow of purple (red and blue); and vice versd, in all 

 instances. 



