The Michelson interferometer can be used to measure the index of refraction of a gas by placing an evacuated transparent tube in the light path along one arm of the device. Fringe shifts occur as the gas is slowly added to the tube. In this problem one fringe shift is defined to mean from a bright pattern to the next dark pattern and not bright to dark and back again to a bright, which would be two fringe shifts. These fringe shifts occur because the wavelength of the light changes inside the gas filled tube. Assume that 621 nm light is used, the tube is 4.63 cm long, and that 326 fringe shifts occur as the pressure of the gas in the tube increases to atmospheric pressure. Calculate ngas, the index of refraction of the gas.