counter-action to the influence of the light. A corresponding effect
occurs when one suddenly exposes to darkness the eye adapted to light.
One can easily observe what goes on then, if, after looking for a time
at an undifferentiated light surface such as the evenly luminous sky,
one covers the opened eyes with the hollowed hands. It will then be
found that the space before the eyes is filled by a sort of white
light, and by paying close attention one recognizes that it streams
from the eyes out into the hollowed space. It may even be several
minutes before the field of vision really appears black, that is,
before the activity of the inner light in the choroid has so far died
away that equilibrium prevails between the non-stimulated nerves and
the non-stimulated blood.

With this insight into the twofold nature of the process of vision we
are now able to describe more fully the negative after-image. Although
in this case, as Goethe himself remarked, the ordinary explanation
seems to suffice, yet in view of our later studies it may be well to
bring forward here this wider conception.

On the basis of our present findings it is no longer enough to trace
the appearing of the after-image solely to a differential fatigue in
the retina. The fact is that as long as the eye is turned to the bright
window-pane a more intensive blood-activity occurs in the portions of
the eye's background met by the light than in those where the dark
window-bar throws its shadow on the retina. If the eye so influenced is
then directed to the faintly illumined wall of the room, the difference
in the activity of the blood persists for some time. Hence in the parts
of the eye adapted to darkness we experience the faint brightness as
strongly luminous, even dazzling, whereas in the parts more adapted to
light we feel the same degree of brightness to be dark. That the action