the impressions of white or black are of a special character. A closer
inspection of these conditions reveals a property of our act of seeing
which has completely escaped scientific observation, but which is of
fundamental importance for the understanding of optical phenomena
dynamically.

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.

The usual interpretation of this phenomenon, namely, that it consists
in a subjective 'contrast' impression of the eye - a red surface in red
light looking brighter, a green surface darker, than its surroundings,
and thereby causing the illusion of white or black - is a typical
onlooker-interpretation against which there stands the evidence of
unprejudiced observation. The reality of the 'white' and the 'black'
seen in such cases is so striking that a person who has not seen the
colours of the objects in ordinary light can hardly be persuaded to
believe that they are not 'really' white or black. The fact is that the
white and the black that are seen under these conditions are just as
real as 'ordinary' white and black. When in either instance the eye
registers 'white' it registers exactly the same event, namely, the