Frits Zernike traced the idea to unwanted companion lines of a diffraction grating. Around 1930 he found that visible striping disappeared when a telescope focused exactly on the grating. A phase strip placed over the principal spectral line changed its relation to the companion light and restored contrast. He then applied the separation of direct and diffracted light to a microscope and demonstrated it at Zeiss in 1932.[ref]
How I discovered phase contrast by Frits Zernike. How I discovered phase contrast, pp. 239–243: grating observations, phase strips and microscope demonstration
His later absorbing and annular strips improved contrast and the appearance of halos; commercial Zeiss accessories followed in 1941.[ref]
How I discovered phase contrast by Frits Zernike. How I discovered phase contrast, pp. 245–246: absorbing strips, annular strips and commercial production
The route from a grating defect to biological observation illustrates why apparently unwanted instrumental effects can be productive experimental clues. Articles by others on the same topic
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