A method of measuring the refractive indexes of liquids in the infrared is described. A Perkin-Elmer spectrometer is adapted for this purpose without interfering with its normal operation as a routine instrument. Although use is made of a Fabry-Perot etalon, the method is not interferometric in the accepted sense. The accuracy that has been obtained is 0.002 at a wavelength of 1µ, but in principle the possible accuracy of the method may be equal to that of the best goniometer.
JOSEPH H. JAFFE, "The Measurement of Refractive Indexes of Liquids in the Infrared," J. Opt. Soc. Am. 41, 166-169 (1951)