Which product design innovation in microscope technology allowed scientists to first observe living cells without destroying them?

The microscope has been a fundamental tool in scientific discovery for centuries, revolutionizing our understanding of the world at the microscopic level. As product design evolved, so too did the capabilities of this essential scientific instrument. This trivia question challenges your knowledge about a critical innovation in microscope design that dramatically expanded scientific research possibilities.

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Phase contrast microscopy, developed by Frits Zernike in the 1930s
Electron microscopy, developed by Ernst Ruska in the 1930s
Fluorescence microscopy, developed by Otto Heimstädt in 1911
Dark-field microscopy, developed by Richard Zsigmondy in the early 1900s