The Refractive Index of Air in the Visible and Photographic Infra-Red

The refractive index of dry C${\mathrm{O}}_2$ free air has been measured from 5300A to 11177A by means of a Fabry-Perot interferometer with invar separating posts. The following dispersion curve was obtained for standard conditions ($\lambda$ in microns): $(n_0-1){10}^7=2885.59+\frac{15.77}{{\lambda }^2}+\frac{0.421}{{\lambda }^4}.$ A comparison of the present results with those of other observers shows that this curve is higher by about 10 parts in ${10}^7$, and that all curves are nearly parallel. This suggests that the discrepancies are due to systematic errors providing, as seems to be the case, that the constitution of air remains sufficiently constant. In the present experiments systematic errors due to phase change at reflection, methods of purification of the air, and bulk modulus of the invar posts were considered. A calculation concerning the oxygen absorption band at 7600A indicates that this band has no measurable effect on the dispersion of air.