An investigation has been made of the extent to which the relative refractivity of the atmosphere is invariant under changes in pressure and humidity. To achieve the necessary precision a refractometer with double-passed chambers 14.3 m long was designed for use in the photographic region. No fiducial line is employed. The relative refractivity of dry air was found to vary by less than 3 × 10-6 over the region from 3890 to 6440 Å for a change in pressure of ½ atm. The relative refractivity of water vapor at low pressure in the region from 3610 to 6440 Å was found to be
δ¯H2O=0.961646+0.0086062 k2-0.00010547 k4+0.00001312 k6,
where k is the vacuum wave number in µ-1.
KENT E. ERICKSON, "Investigation of the Invariance of Atmospheric Dispersion with a Long-Path Refractometer," J. Opt. Soc. Am. 52, 777-780 (1962)