Acoustic Spectrum of Liquids

As is now well known, examination by means of a Fabry-Perot étalon reveals remarkable changes in the spectral character of monochromatic radiation when it is scattered within a dust-free liquid. Earlier studies in this laboratory¹ disclosed that the relative intensity of the undisplaced central line to that of the two Doppler-shifted companions appearing on either side of it varies greatly from liquid to liquid, being for example much larger in carbon tetrachloride than in toluene. The origin of the undisplaced central line has been until now one of the unsolved problems of the subject; its high intensity in the case of carbon tetrachloride and its practically complete polarization in this liquid as well as in toluene and carbon disulphide indicate that it cannot be ascribed to the depolarized scattering by optically anisotropic molecules.