Refractive index of He in the region 920–1910 Å

The refractive index of He has been determined in the region 920–1910 Å by measurements of wavelength shifts in a 3-m spectrograph alternately filled with He and evacuated. Differential pumping systems were used to allow operation of the light source at conveniently low pressures. Several plates were measured and analyzed in order to reduce statistical errors. The results (n − 1)NPT = 4.85 × 10−5 at 919 Å agree with the theory within 1%, i.e., less than the experimental error.