This hypothesis was tested by 
Hans Wallach of Swarthmore 
College in the following way. In 
a dark room, one slide projector 
produced a disk on a screen and 
another projector produced a 
ring around the disk, as shown 
in the figure to the left. 
Everything else was masked out 
on each slide. When Wallach 
held the intensity of the disk 
constant and varied only the 
intensity of the ring, observers 
perceived the lightness of the 
disk to vary all the way from 
white to black. It looked white 
when the luminance of the 
ring was about one half that of 
the disk, it looked gray when 
the luminance of the ring was 
about twice that of the disk, 
and it looked black when the 
ring-to-disk ratio was about 30 
to 1.

Dark-room experiments on 
lightness perception: (left) by 
varying the intensity of light 
(or luminance) in the 
projection of the ring, the disk 
inside it will appear to vary in 
lightness from white to black. 
(Right) experiment 
demonstrating that the ratio of 
luminances of adjacent 
surfaces determines perceived 
lightness: in the illustration 
the disks do not appear to be the 
same shade of gray because the 
regions surrounding the rings 
and disks also affect our 
perception of the shades.

    This last effect is astonishing 
when one considers that the 
disk is not a dark surface but a 
white screen illuminated by a 
single projector. With the ring 
turned off, the disk looks like a 
bright, luminous source of 
light.