The Poggendorff Illusion
The two oblique lines in the 
figure on the left do not appear 
to be aligned although they in 
fact are. The dashed line in the 
figure on the right represents 
where subjects typically place a 
line that they perceive to be 
aligned with the solid lower 
oblique line. The separation 
between the dashed line and 
the upper solid line is a 
measure of the illusion.

In the illustration at left, the 
illusion created by J. C. 
Poggendorff in 1860 shows two 
oblique test lines crossing 
parallel vertical inducing 
lines. The illusory effect is that 
the oblique lines, which are in 
perfect alignment, do not 
appear to be. The upper line 
appears raised with respect to 
the lower one. For the oblique 
lines to appear to be in 
alignment, a lower oblique line 
(such as the dashed line in the 
right-hand figure) would have 
to be substituted for the upper 
one.