This article concerns the scientist. For the unrelated string quartet, see Zoellner Quartet.
Johann Karl Friedrich Zöllner (8 November 1834, Berlin - 25 April 1882, Leipzig) was a German astrophysicist who studied optical illusions.
He invented the Zöllner illusion where lines that are parallel appear diagonal. He also successfully proved Christian Doppler's theory on the effect of motion of the color of stars, and the resulting shift of absorption lines, via the invention of a very sensitive spectroscope which he named "Reversionspectroscope". He had shown also that the red-shift was in addition caused by variation in the stars' lights intensities with the help of his "Astrophotometer". He made the first measurement of the Sun's apparent magnitude. His result was very good, less than 0,1 from modern value.
The lunar crater Zöllner is named in his honor.
Zollner first became interested in Spiritism in 1875 when he visited the scientist William Crookes in England. Zollner wanted a physical scientific explanation for Spiritism and came to the conclusion that physics of a four-dimensional space may explain spiritualism. Zollner attempted to prove that spirits are four-dimensional and set up his own experiments with the medium Henry Slade. These experiments were recorded by Zollner in a book titled Transcendental Physics in 1878. According to Zollner the experiments were a success. Very few scientists however were convinced by Zollner's experiments. Critics have pointed out that the medium Henry Slade was a fraud who resorted to trickery and that he had cheated on the experiments.
Martin Beech in The Physics of Invisibility: A Story of Light and Deception wrote:
Zollner presented his book as a physical investigation into the paranormal, including clairvoyance, matter transfer, ghosts and natural spirits. The main thrust of Zollner's argument was that all these kinds of mystic phenomena could be explained if ghosts (spirits of the dead) inhabited a "real external world" composed of four spatial dimensions. On this basis, he argued that what we see as ghosts are really just shadows projected into our three dimensional spatial realm.