The gas cell is positioned in one arm of the interferometer between the beam splitter and the moveable mirror such that the surfaces of the cell are perpendicular to the laser beam. After obtaining a clear set of fringes, pump some of the air out of the cell. Measure the pressure before and after and the number of fringes passed while pumping. The refractive index of the gas varies directly with its density, and the index of refraction of a vacuum is one. Thus a graph of refractive index as a function of pressure must go through the point x=0, y=1. Determine the slope of the line by calculating the change in the refractive index: if n1 is the index at the initial pressure and n2 is the index at the final pressure, then n1-n2=mL/2d, where m is the number of fringes counted, L is the wavelength, and d is the length of the gas cell (only the gas region, not the end plates).
The final photograph above is a detail of the fringe pattern. In the video linked below, you can see the fringes shift as air is pumped from the gas cell. You can hear the clicks from the hand pump as it removes air from the cell.