Johann Karl Friedrich Zöllner (1834 – 1882) was a German astrophysicist who studied optical illusions. He invented the Zöllner illusion where lines that are parallel appear diagonal. The lunar Zöllner crater is named in his honor. 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" (1865). Zollner became convinced in the reality of a fourth dimension after succumbing to the spiritualist trickery of the notorious medium Henry Slade.[1]