Johann Karl Friedrich Zöllner

Johann Karl Friedrich Zöllner (8 November 1834, Berlin – 25 April 1882, Leipzig) was a German astrophysicist who studied optical illusions. He was also an early psychical investigator.

From 1872 he held the chair of astrophysics at Leipzig University. He wrote numerous papers on photometry and spectrum analysis in Poggendorff's Annalen and Berichte der k. sächsischen Gesellschaft der Wissenschaften, two works on celestial photometry (Grundzüge einer allgemeinen Photometrie des Himmels, Berlin, 1861, 4to, and Photometrische Unterstuchungen, Leipzig, 1865, 8vo), and a curious book, Ueber die Natur der Cometen (Leipzig, 1872, 3rd ed. 1883).

He discovered 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 the modern value.

The lunar crater Zöllner is named in his honor.

Zöllner first became interested in Spiritualism in 1875 when he visited the scientist William Crookes in England. Zöllner wanted a physical scientific explanation for the phenomena and came to the conclusion that physics of a four-dimensional space may explain spiritualism. Zöllner attempted to demonstrate that spirits are four-dimensional and set up his own séance experiments with the medium Henry Slade which involved slate-writing, tying knots on string, recovering coins from sealed boxes and the interlinking of two wooden rings. These experiments occurred in November and December 1877 at Zöllner's home in Leipzig. He invited the scientists Wilhelm Eduard Weber, Gustav Fechner, Wilhelm Scheibner and Wilhelm Wundt to some of the sittings.

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