Referring to Fig. 1, 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 Δ"L" ≈ 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.