The Poggendorff illusion and the visual arts

The discovery of geometric-optocal illusions is generally considered an important milestone in the early development of scientific psychology. However, it is highly likely that many of such illusions were already known to visual scientists before the XIX century. Here we show that first awareness of the Poggendorff illusion actually dates back to the V century A.D., when an unknown artist was creating a large mosaic known as "Lunetta di San Lorenzo" in the Mausoleum of Galla Placidia in Ravenna. During the creation of the mosaic the artist became aware of a visual distortion that disrupted the integrity of a long cross carried by the saint on his right shoulder. We demonstrate that the artist purposely sought a geometrical misalignment that restores the perceived integrity of the cross when the mosaic is viewed from ground level inside the mausoleum. This fact strongly suggests that the artist was aware of the psychological nature of the distortion. Finally, we discuss cases in which the illusion was or was not corrected for in more recent artworks.