  The Poggendorff illusion in its classical form of parallel lines interrupting
a transversal is viewed from the perspective of being related to the everyday
experience of observing the light refraction in water. It is argued that, if
one considers a transversal to be a light ray in air and the parallel lines to
form an occluding strip of a medium with the refractive index being between
that of air and water, then one should be able to account, both qualitatively
and quantitatively, for most of the features associated with the Poggendorff
illusion. Statistical treatment of the visual experiments conducted with 7
participants, each analysing 50 configurations having different intercepting
angles and strip widths, resulted in the effective refractive index of the
occluding strip N = 1.13 +/- 0.15, which is sufficiently close to the average
(between that of water and air) refractive index of ~ 1.17. It is further
argued that the same mechanism can also be employed to account for many
variants of the Poggendorff illusion, including the corner-Poggendorff pattern,
as well as for the Hering illusion.
