A high‐sensitivity high‐resolution system for the study of light scattered from thermal fluctuations in gases has been constructed. A Bendix Channeltron photomultiplier tube with a dark noise of three‐tenths of a photoelectron/second is the basis of an experimental setup which includes a Fabry‐Perot interferometer having a bandwidth of 10 MHz with a piezoelectric scan and a frequency stabilized single‐mode He‐Ne laser. Resolution of 10 MHz is, therefore, attainable, and signal levels as low as six‐tenths of a photoelectron/second yield reliable data. With this apparatus the density‐density correlation function,G(r,t), of several gases has been investigated for a range of pressures. Whether the form ofG(r,t) is derived from a kinetic or hydrodynamic view depends on the value of the Knudsen number, i.e., the ratio of λs, the wavelength of the scattering fluctuation, to the mean free path. Previous studies have concentrated on varying the angle of scattering (changing λs) while this study varies the gas pressure itself (changing the mean free path), allowing a more detailed investigation of the transition from the kinetic to the hydrodynamic view.