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
atm. The relative refractivity of water vapor at low pressure in the region from 3610 to 6440 Å was found to be
where k is the vacuum wave number in μ−1.
© 1962 Optical Society of America
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