  Quantitative measurements of the vibrational eigenmodes in ultra-high-Q
silica microspheres are reported. The modes are efficiently excited via
radiation-pressure induced dynamical back-action of light confined in the
optical whispering-gallery modes of the microspheres (i.e. via the parametric
oscillation instability). Two families of modes are studied and their frequency
dependence on sphere size investigated. The measured frequencies are in good
agreement both with Lamb's theory and numerical finite element simulation and
are found to be proportional to the sphere's inverse diameter.
