The refractive index of gaseous heavy water

The refractive index of heavy water, 100% pure, was found by Lewis and Luten, and Luten to be less than that of normal water by 0·00456 at 20°C for λ5893, and this result has been approximately confirmed by the work of Washburn, Smith, and Frandsen, Tilton and Taylor (J. K.), and Taylor (H. S.) and Selwood. Luten, and Tilton and Taylor also measured the dispersion of D2 O, and found that the curve was slightly flatter than that of normal water. Lutten gives the figures shown in Table I. So large a diminution of the normal index is surprising, for, even if we assume that the D atom is identical with the H atom in refractive effect, and allow for the increase in the molecular volume of the D2 O molecule above that of H2 O, the change is greater than would be expected.