A study of the comparative method of determining gaseous refractivities

It is shown that the comparative method for the determination of refractive indices by white-light interferometry yields the ratio of the group refractivities and not the ratio of the phase refractivities. The effective wave-length of the ordinary gas-filled lamp was found to be close to that of sodium light. For ordinary gases, however, any wave-length in the same region may be taken without serious error. Observation of the shift of the central achromatic fringe due to differences of dispersion in the two paths enables the difference of dispersion to be measured. This enables the group index to be corrected and the phase index calculated at any point in the visual range of the spectrum. The method has been applied to several common gases and to mixtures of the vapours of volatile organic solvents with air, and appears to give results comparing not unfavourably in accuracy with those obtained by the absolute method in monochromatic light.