Georges Sagnac

“ A uniform clockwise rotation of the interferometer produces a counter-clockwise aether wind; it delays by x the phase of the beam (T) , whose circulation around the area S is clockwise, and it advances the opposite beam R by an equal amount, thus causing a fringe displacement of 2x. The displacement z, that I observe comparing an s image with a d image, must be double that of the former. [2] On the basis of the value of x observed previously (loc. cit. 1910 and 1911) , we have
where indicates the speed of light in vacuum, and λ is the wavelength used. ”

“ I point out that perfect superposition of the two opposite beams T and R is characterized by general extinction of the telescope field for the radiation employed, which is here close to the indigo portion of the mercury arc spectrum. Starting from there, a small rotation ε of the beam splitter around a vertical axis, in the clockwise direction (swing-direction D [2] ) , or counter-clockwise (swing-direction S [3] ) , narrows the dark field into a central vertical fringe bordered by parallel side fringes. ”

“ In a horizontal optical circuit, at latitude α, the diurnal rotation of the Earth must, if the aether is motionless, produce a relative aether vortex whose density is or radian per second, where T is the duration of the sidereal day ”

“ I gradually start up an electrical motor whose vertical axis carries a horizontal disc D that is encircled by a leather belt and which rolls this belt on the thick rim of the plate. When the desired rotational frequency N is reached, I make a photographic exposure by establishing electric current to the small lamp O, using a sliding contact on the axis of the revolving plate. ”

“ The interference displacement z, a constant fraction of the fringe spacing for a given rotation frequency N, becomes invisible on the photographs when the fringes are adjusted to be sufficiently narrow ”

“ Second method. - I measure directly the distance from the vertical central fringe of the d image to the central fringe of the s image, which is exactly adjacent to the first below a sharp, horizontal separation-line. ”