English: The Kennedy-Thorndike experiment. Key optical components were mounted within vacuum chamber V on a fused quartz base of extremely low coefficient of thermal expansion. A water jacket W kept the temperature regulated to within 0.001°C. Monochromatic green light from a mercury source Hg passed through a Nicol polarizing prism N before entering the vacuum chamber, and was split by a beam splitter B set at Brewster's angle to prevent unwanted rear surface reflections. The two beams were directed towards two mirrors M₁ and M₂ which were set at distances as divergent as possible given the coherence length of the 5461 Å mercury line (≈32 cm, allowing a difference in arm length ΔL ≈ 16 cm). The reflected beams recombined to form circular interference fringes which were photographed at P. A slit S allowed multiple exposures across the diameter of the rings to be recorded on a single photographic plate at different times of day.