There are well-known scientific toys illustrating this law [of complementary colour]. A short word, printed in large red letters, is looked at for half a minute. The eyes are shut and an image of the same word appears, but the lettering is green. Many such experiments may be made in the open garden. The brilliant orange African Marigold has leaves of a rather dull green colour. But look steadily at the flowers for thirty seconds in sunshine and then look at the leaves. The leaves appear to be bright blue!