Earlier studies of the Sagnac effect

Studies of the Sagnac effect carried out during the latter part of the nineteenth and the first half of the twentieth century are reviewed. A discussion of the chronology issue shows, in particular, that O. Lodge was the first to recognize the possibility of the effect. It is also shown that, apart from detecting rotation, in most studies the improvement of the Fresnel—Fizeau drag coefficient in rotating reference frame was the primary concern. As possible directions for the development of Sagnac interferometry, a wider operational range for electromagnetic waves, the interference of material de Broglie waves, and the interference of acoustic and magnetic surface waves are considered.