Georges Sagnac

“ During many tests, I constantly observed the predicted fringe displacements. The fact that effect z reverses when I turn the beam-splitter by only a fraction of degree to reverse the swing-direction of its adjustment, characterizes the effect as a phase difference related to the motion of the interferometer, and makes it possible to distinguish it from the effect of deformation of the optical components. ”

“ 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. ”

“ 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. ”