It is well known that a corporeal surface, which we experience as white, has the characteristic of throwing back almost all the light that strikes it, whereas light is more or less completely absorbed by a surface which we experience as black. Such extreme forms of interplay between light and a corporeal surface, however, do not only occur when the light has no particular colour, but also when a coloured surface is struck by light of the same or opposite colour. In the first instance complete reflexion takes place; in the second, complete absorption. And both these effects are registered by the eye in precisely the same manner as those mentioned before. For example, a red surface in red light looks simply white; a green surface in red light looks black.