The object of this investigation was to test experimentally the assumption of electromagnetic dispersion theory that the natural period of vibration of the resonators of a medium remains constant with variations in pressure. Two dilute aqueous solutions of uranine of slightly different concentrations were studied in the high pressure spectrometer. Refractive indices for the three principal lines of the mercury arc, and for the wave-length corresponding to the absorption band maximum (4390A) were determined for pressures ranging up to 915 kg per cm2 in one solution, and up to 1823 kg per cm2 in the other. It was found that the absorption band retained its position relative to the rest of the spectrum formed by the liquid prism under various pressures. It is concluded that the natural frequency of vibration of the resonators is affected to no measurable extent by the pressure or density of a medium.
WILLIAM JAMES LYONS, "The Behavior of the 4930A Absorption Band of Uranine Solutions Under High Pressure," J. Opt. Soc. Am. 26, 144-145 (1936)