  We present a method to measure the refractive index of gas at 633 nm
absolutely, which does not need filling or pumping gas during the measurement.
We develop a quasi-synthetic wavelength (QSW) theory by means of the
configuration of two-frequency Jamin interferometry and vacuum tubes with
specific lengths. With the aid of the QSW theory, we construct a gas
refractometer and demonstrate its performance by the measurement of dry air and
nitrogen gas at different pressures ranging from 80 kPa to 100 kPa. The results
indicate that the refractometer has an uncertainty of better than 1E-7 and a
dynamic range of 3.95E-4.
