Based on this experiment, we 
can conclude that light-
intensity ratios yield the 
varying shades of lightness. But 
what about the problem of 
constancy? The reader may 
have already anticipated that 
the ratio hypothesis also 
elegantly explains constancy. 
When the prevailing 
illumination changes, it 
affects the absolute luminance 
of every surface but it does not 
affect the ratios among them. 
To test the hypothesis 
rigorously, Wallach introduced 
a second ring-and-disk pair, as 
shown at left. Suppose the
absolute values in the first 
pair are Ring: 2 and Disk: 1, 
where the values are arbitrary 
units and where the disk 
appears to be a particular value 
of gray. Suppose the absolute 
value of the ring in the second 
pair is 8. The observer is now 
given the task of adjusting the 
luminance in the disk of the 
second pair until it appears to 
have the same color as the disk 
in Pair 1. The result, averaging 
for all subjects over many 
trials, was a setting very close 
to 4, meaning that observers 
selected a disk whose 
luminance was 4 times as great 
as that of the disk in Pair 1 to 
which it was being matched. 
More importantly for our 
purposes, the result is quite 
close to the predicted value of 4 
based on the ratio of 2 to 1 in 
Pair 1. Thus the hypothesis is 
confirmed.

This phenomenon is 
sometimes referred to as the
Hermann grid.  It clearly 
demonstrates that inhibitory
processes within the retina
influence effects of contrast,
and also lightness constancy.
However, higher-order
processes are undoubtedly 
involved as well.