Michelson and Morley directed a beam of white light toward a half-transparent mirror—also known as a beam-splitter—so that the beam transmitted through the glass and reflected off the mirror, like the swimmers going across the river and upstream. Two additional mirrors directed each half-beam back toward the beam-splitter. In turn, the beam-splitter directed the beams onto a screen.
If the half-beams reached the beam-splitter at the same time in this final step, then the crests and troughs of the two light waves reaching the screen would align. If one of the half-beams "won the race," then the crests and troughs of the two light waves would no longer align.
As the scientists could not know the ether's direction, the entire apparatus could be rotated to various positions. As it spun, the two light waves would change from aligned to misaligned.
