By a simple experiment, the 
edges where the folded sides of 
palm leaf meet can be 
"flattened out" perceptually so 
that the surfaces look like as 
though they are in the same 
plane, and in this way 
constancy can be abolished. To 
perform this experiment: (1) 
close one eye, (2) hold the head 
still, and (3) hold up cardboards 
to block from view the top and 
bottom of the picture, showing 
only a middle strip. The change 
will be dramatic: what before 
looked like different surfaces 
now will look like different 
shades of green. The results of 
this experiment suggest that if 
and only if the two regions that 
differ in luminance appear to 
be in the same plane does the 
ratio between them lead to 
differing lightnesses. If this is 
true, it is clear that luminance 
ratio does not so automatically 
determine lightness perception 
as has been implied.

    Experiments by Alan Gilchrist 
at Rutgers University have now 
established that, when the 
regions that form luminance 
ratios are in different 
perceived planes, even parallel 
planes, those ratios will have 
little bearing on lightness 
perception. Yet, when these 
regions are perceived to be 
adjacent and in the same plane, 
the ratios do govern such 
lightness perception. How can 
we make sense of these facts?