= Zernike's discovery of phase contrast
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<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.<How I discovered phase contrast by Frits Zernike>[How I discovered phase contrast, pp. 239–243: grating observations, phase strips and microscope demonstration]{ref}
His later absorbing and annular strips improved contrast and the appearance of halos; commercial Zeiss accessories followed in 1941.<How I discovered phase contrast by Frits Zernike>[How I discovered phase contrast, pp. 245–246: absorbing strips, annular strips and commercial production]{ref} The route from a grating defect to biological observation illustrates why apparently unwanted instrumental effects can be productive experimental clues.
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