warrant. Finally, the test lines 
that are oriented vertically in 
the illustration at left ought to 
create as much of an illusion as 
the test lines in the original 
illusion, if depth processing 
were the whole story. As can be 
seen, however, the illusion is 
diminished or nonexistent 
here.

According to the depth-
processing theory, the unequal 
horizontal lines ought to look 
more nearly equal in length 
than they actually do.