Shown below is a schematic picture of the apparatus. A laser is directed toward a distant mirror M1, which returns the beam to a photodetector D1. A beam splitter (glass plate) reflects a fraction of the initial beam into detector D2. The detectors give an output voltage which varies with the intensity of the incident light. An oscillator is connected to the laser which causes its brightness to vary sinusoidally (called modulation). This, in turn, causes the voltage coming from the detectors to vary sinusoidally. The signal from D1 will be delayed by the time of travel of the light to M1 and back. The time difference between the signals at D1 and D2 is measured by comparing the two sine waves on an oscilloscope.
The actual experimental setup and equipment are slightly more complicated. The beam travels down the hallway to the north end of the science building just below ceiling height - 82'' off the floor. There are some mirrors not shown in the schematic and some lenses to focus the beams on the detectors. The detectors are mounted in the same box with their associated signal amplifiers and connections.