Refractive Index ofHe4: Saturated Vapor

The refractive index $n$ of saturated helium vapor has been measured from 1.5 to 4.2°K, by means of a Jamin interferometer and a metal cryostat with optically flat windows. About 200 experimental points were taken. For the Hg green line ($\lambda =5462.27\mathrm{}$ A), ($n-1\mathrm{}$) increases smoothly from (31±2)×${10}^{-6\mathrm{}}$ at 1.5°K to (3300±15)×${10}^{-6\mathrm{}}$ at 4.2°K. Eight separate absolute determinations of ($n-1\mathrm{}$) were made over this temperature range and the results were combined with the calculated vapor density to give the polarizability per mole, $N_0\alpha$. The mean polarizability is 0.1245±0.0005 ${\mathrm{cm}}^3$/mole. Individual determinations from 1.8° to 4.2°K do not deviate from this by more than 0.0008. This value should be compared with 0.1246±0.0002 ${\mathrm{cm}}^3$/mole which may be calculated from the measurements of ($n-1\mathrm{}$) for helium gas at N.T.P.