Johann Karl Friedrich Zöllner

Johann Karl Friedrich Zöllner (8 November 1834 – 25 April 1882) was a German astrophysicist who studied optical illusions. He was also an early psychical investigator. He discovered the Zöllner illusion where lines that are parallel appear diagonal.

In astronomy, he proved Doppler effect in stars and made the first measurement of the apparent magnitude of the Sun.

He is also known for his scientific experiments about spiritualism, studying the work of the medium Henry Slade.

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