In middle age Zöllner encountered Slade on one of his European tours and was impressed by Slade's ability to get spirits of the dead to write messages on school slates with ordinary chalk. Slade very rarely fooled university professors in the sciences with this simple sleight-of-hand stunt, and when he fooled Zöllner completely, he recognized he was sitting on a gold mine as far as getting academic publicity. In conversations with the already somewhat senile astronomer, whose knowledge of physics and mathematics was antiquated even by the standards of the late 1870s, Slade discovered that the professor was mainly interested in "the incredible fourth dimension." Soon, unsurprisingly, Slade was demonstrating simple magic tricks with wooden hoops, rope, string and pieces of furniture... tricks which he claimed were demonstrations of his ability to make use of the "fourth dimension" to pass one solid three-dimensional object through another. Zöllner was impressed, and promptly sat down to write one of the most sublimely insane of all pseudoscience books, TRANSCENDENTAL PHYSICS, mainly a description of Slade's miracles, interspersed with deranged raving about higher physical dimensions and the Spirit Realm which must lay somewhere in there. According to L. Sprague de Camp, Zöllner's behavor became so bizarre during and just after the publication of TRANSCENDENTAL PHYSICS that he was confined to a mental institution for the short remainder of his life. Thus, he literally became the "mad scientist" so popular in 1920s-1930s pulp fiction— a rare example, since scientists as a general rule are disgustingly sane.

What about the Zöllner illusion itself? This is one of those illusions that is in every book and every casual Internet collection of illusions, and that actually is generally displayed in a form that doesn't work! Which of the examples above works best for you? The illusion, in case you can't see it at all, is that the parallel straight lines may appear bent as a result of the short "spines" sticking out of them at a 45 degree angle.

Might there be some relation between the Zöllner illusion and the so-called Cafe Wall illusion? Try the interactive versions of the classic Zöllner version here to see if you can find an arrangement in which the illusion actually works: Aceviper | Cut the Knot | Sandlot Science | Hudler.