The original Müller-Lyer 
illusion.

As can be seen in the 
illustration to the left, the line 
contained within reversed 
arrowheads appears appreciably 
longer than the line of equal 
length contained within the 
normal arrowheads. Hundreds 
of papers have been published 
on this illusion, and some 
interesting facts about it have 
been uncovered, but in my 
opinion it has not yet been 
satisfactorily explained. 
Several entirely different 
factors may cooperate to 
produce it.