The Bisection Illusion

Optical illusions are intimately related to perspective.  Many judgements made in paintings require an evaluation of size and form, and optical illusions can often impede this process.  The purpose of this section is to highlight the existence of numerous subtle, but very real, optical illusions.

Consider an arrangement shown in the Fig. 59.  Two lines are drawn such that the horizontal line is bisected by the vertical line. 

Fig. 59 The bisection illusion.  The arrangement on the left
 has two lines of the same length.  The arrangement on the right
has the vertical line much shorter than the horizontal one. 

On sight, it appears that the arrangement on the right uses lines of the same length.  In fact, the vertical line is about 12.5% shorter than the horizontal one.  The arrangement on the left uses lines of the same length, but the vertical line appears much longer than the horizontal one.

Many explanations have been suggested.  A strong argument is that this is due to the curvature of the retina.  Another is that the act of halving the horizontal line makes us lose sense of its full extension.  A weaker argument is that our eyes are placed in a horizontal line, so we tend to assess horizontal and vertical lengths differently.