A Michelson twin interferometer for precise measurements of the refractive index of gases between 100 K and 1300 K

A Michelson twin interferometer is described, which is used for precise measurements of the refractive index n of gases in the visible wavelength range and temperatures between 100 K and 1300 K. The evacuated twin interferometer (length 2.4 m, diameter 90 mm) consists of two adjacent identical Michelson interferometers A and B, using the same beam splitter and mirrors. Interferometer A serves as the proper measuring interferometer with the gas sample cell in measuring beam 1 and the identical, but evacuated, reference cell in its reference beam 2. Interferometer B is used only for controlling the geometrical changes of the interferometer equipment during the refractive index measurements performed with interferometer A. Both the gas sample cell and the reference cell are made from quartz glass and thermostated. The construction of this very stable twin interferometer as well as the devices for gas pressure and temperature measurements are described in detail.