Münsterberg's Poggendorff Arch
from Michael’s Visual Phenomena & Optical Illusions
Above you see one of the beautiful pointed Gothic arches from the Freiburg Münster (where I live – picture a little modified).
It’s shape, however, is somewhat marred by the vertical gray bar. Furthermore, the two stone arcs going up behind it clearly are not going to meet at the top. Or are they?
Press the “←→” button: misalignment switches around. Or grab the bar with your mouse and move it around. Or use the slider on the right and make the bar a little transparent.
Reminded of this by one of Richard Wiseman’s witty movies, I interpreted it as a strong varient of the Poggendorff illusion. Bernd Lingelbach (thanks!) traced it down and found the neighbouring picture in Walter Ehrenstein’s 1954 book (p 154, Fig. 100), designated as: Münsterberg’s modification of Delboeuf’s variant of the Poggendorff illusion… So, since Delboef already has his own illusion :), I will call it “Münsterberg’s Poggendorff Arch” or “Disjointed Arch”.
The effect is really strong for me, e.g.: After building the demo above, I looked at the figure on the right, and thought: this can’t be true! So I copied & mirrored the left part and overlaid it in gray – yes, they do meet. Try yourself by moving the mouse over the picture.
Ehrenstein WA (1954) Probleme der ganzheitspsychologischen Wahrnehmunglehre. 3. Auflage, JA Barth-Verlag, Leipzig.