A new and much improved displacement interferometer has been built after the general plan of the one previously described to measure simultaneously refractive index and dispersion constants for a gas over a range of pressures above atmospheric. It is constructed as a single unit on a steel I-beam framework 10 feet long and 2 feet wide. The whole apparatus is supported on automobile inner tubes which very effectively eliminate vibration. The features also include a G.E. S-1 sunlamp light source, aluminum mirrors, a carefully calibrated micrometer screw of high quality and an improved temperature control.

Observations made on nitrogen at four wave-lengths, 5780A, 5461A, 4359A and 4047A, corresponding to pressure runs up to 14 atmospheres measured by a Keyes dead-weight gauge, at temperatures of 50°C, 30°C and 0°C, give readings that are linear with pressure to such a degree that the Cauchy dispersion constants can be expressed to four significant figures. The results for N.T.P. conditions are: $A_0-1=0.0002932\mathrm{}$ and $B_0=1.637\mathrm{}\times {10}^{-14\mathrm{}}$. Since $A_0-1\mathrm{}$ is the extrapolated infinite wave-length refractive index, a dielectric constant $\epsilon =1.0005864\mathrm{}$ is thus predicted. This agrees with published determinations. The refractive index at any wave-length can be computed from these results and values so calculated are found to agree with values found by other methods, which are not suitable for direct extrapolation to infinite wave-length and which are not likely to be so accurate.

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