Johann Karl Friedrich Zollner

Johann Karl Friedrich Zöllner (1834 – 1882) was a German astrophysicist who studied optical illusions.

He introduced the Zöllner illusion where lines that are parallel appear diagonal.

The Zöllner crater on the Moon is named in his honor. For the first time he actually proved Christian Doppler's theory on the effect of motion of the color of stars, and the shift of absorption lines thereby (red-shift) (1869), by the invention of a very sesitive 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" (1865).

Zollner became convinced in the reality of a fourth dimension after succumbing to the spiritualist trickery of the notorious medium Henry Slade.