• 1868: French physicist Armand-Hippolyte-Louis Fizeau suggests masking a telescope aperture to perform interferometry. He proposes measuring the sizes of stars by placing a two-hole mask over a telescope and observing the resulting interference pattern.

• 1876: ¿douard Stephan tries Fizeau's technique with the 80-centimeter telescope at Marseilles. But the 65-centimeter separation between the holes that he uses is not enough to measure the stars' sizes. When Stephan looks through his eyepieces at a star, he sees an image of the star from each aperture in the mask. These images are large and usually overlap. The region of overlap is crossed by dark stripes (interference fringes). To measure a star's diameter, one increases the separation between the apertures until the fringes disappear. The larger the separation required, the smaller the star. But Stephan runs out of telescope before the separation becomes large enough. He concludes only that the stars he observes are all smaller than 0.16 arcsecond.
  
  
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