COMPLEMENTARY COLOURS :—COLOURED SHADOWS. 439 
particularly noticed in regard to the complementary 1 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. 
