English: Hammar experiment, 1935. An unpolarized beam of white light is split into two beams of equal intensity. The oppositely directed beams are directed through a heavy walled steel tube whose ends are plugged with lead. Once outside the tube, the oppositely directed beams are reunited to form interference fringes. If any differential velocity exists between the light inside the tube versus that outside the tube, the differential velocity should manifest itself as a fringe displacement.