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Abstract
Since the development of the undulatory theory of light, the measurement of the wave-length of the source of light has played an important part is experimental optics. Practically all measurements made by means of an interferon- meter involve an accurate knowledge of the wave-length of 1 the source. In 1868 Fizeau proposed the application of the interference of light to direct measurements of the diameters of the fixed stars. This method was put into practice by Stephan at the Marseille Observatory in 1873 by using a telescope of 80 centimeters aperture. In all the series of experiments, the apparatus showed only the extreme minuteness of the stellar disks but an angular diameter less than 1/5 of a second of arc could not be distinguished. He proved that in order to measure them a much larger instrument would be necessaгу.
