Basic Concepts.- The Equations Governing Linear Perturbations in a Quasi-Static Star.- Deviations from the Hydrostatic and Thermal Equilibrium in a Quasi-Static Star.- Eigenvalue Problem of the Linear, Isentropic Normal Modes in a Quasi-Static Star.- Spheroidal and Toroidal Normal Modes.- Determination of Spheroidal Normal Modes: Mathematical Aspects.- The Eulerian Perturbation of the Gravitational Potential.- The Variational Principle of Hamilton.-...