In 1952, Frits Zernike won the Nobel Prize in physics for the phase-contrast microscope. Zernike had not created this microscope from scratch. He invented a small but revolutionary addition to the optical system of an existing microscope.
Not a regular microscope
With a regular microscope, you cannot see any detail in transparent objects. A cell in the human body, for example, is made up of a number of transparent parts that refract light differently. By changing the phase of the light - comparable to Polaroid sunglasses - you can make them visible. With this phase-contrast microscope, it was suddenly possible to study living cells.