Apparatus

In order to precisely measure the wavelength of the light source in the MM interferometer, you will need a precise measurement of the distance that the mirror of the interferometer has moved. This precise measurement is achieved using the differential micrometer shown in Figure 4.

Reading the differential micrometer is much like reading a typical caliper. There is a short video here that explains how to read the scale and has an example measurement.

Figure 4: The micrometer has a total length of 2.5 mm, with tick-marks every 0.05 mm along the inner shaft.

The apparatus used to determine the index of refraction of gases is basically a series of valves connecting the gas cell to atmosphere and to a vacuum pump  The gas cell has a length of 100.0(1) mm

The valves can be used to add gas to the container or to evacuate the chamber using the vacuum pump.  The pressure sensor is a piezo-resistive differential pressure transducer that generates a voltage based on the pressure difference between the cell and the atmosphere.  10 V corresponds to a relative pressure of 1 atmosphere and 0 V corresponds to a relative pressure of 0 atmosphere.  If the user measured the change in pressure using the voltage transducer and the number of fringes in the output beam, then the index of refraction of the gas can be determined.