This arrangement is typical of 
that used for an experiment on 
lightness constancy. The 
observer must match a standard 
gray square on one side with a 
comparison square on the other 
side in a very different 
illumination. The match is 
usually to the shade of gray of 
the standard.

    Most students of perception 
believe that the relational 
explanation of perceived 
lightness, lightness constancy, 
and contrast is essentially 
correct. From the discussion so 
far, it appears that the problem 
of lightness constancy is much 
better dealt with by a theory 
based on stimulus relations 
than by a taking-into-account 
type of theory, although, as we 
shall see, there is more to 
lightness constancy than 
stimulus ratios.