A new refractometer by combining a variable length vacuum cell and a double-pass Michelson interferometer

A new refractometer with a variable length vacuum cell has been developed, where the refractive index of air is determined by measuring the changes in the optical path difference between the air of interest and a vacuum as a function of the changes in the cell length. An uncertainty of 4/spl times/10/sup -9/ in the index has been achieved.